The webpage for Fairfax County Public Schools’ online learning platform, shown here in April 2020. (Fairfax County Public Schools)

A report on student grades from one of the nation’s largest school districts offers some of the first concrete evidence that online learning is forcing a striking drop in students’ academic performance, and that the most vulnerable students — children with disabilities and English-language learners — are suffering the most.

Fairfax County Public Schools in Virginia, which has been mostly online since March, published an internal analysis this week showing that, between the last academic year and this one, the percentage of middle school and high school students earning F’s in at least two classes jumped by 83 percent: from 6 percent to 11 percent. By the end of the first quarter of 2020-2021, nearly 10,000 Fairfax students had scored F’s in two or more classes — an increase of more than 4,300 students as compared with the group who received F’s by the same time last year.

Experts have warned since the beginning of the pandemic, and the unexpected national experiment in online learning, that remote schooling would take a serious academic toll on children.

Now, evidence of poor achievement in virtual classrooms is beginning to emerge nationwide: In the Independent School District in Houston, more than 40 percent of students are earning failing grades in at least two of their classes, according to data reported by the Houston Chronicle. Likewise in St. Paul, Minn., where the superintendent recently reported that nearly 40 percent of St. Paul Public Schools high-schoolers have failing marks, local TV station KARE reported.

Educators have struggled with the question of how to grade students in a virtual environment since the spring. Many districts opted for a pass/fail system to close out the final quarter of the 2019-2020 school year because students and teachers had been thrust into an online-only world with almost no time to adjust or prepare.

But the tactic led to significant dips in engagement and attendance, as families forced to navigate the vagaries of the pandemic prioritized other concerns. So, after school districts built remote learning curriculums from scratch over the summer, many advised teachers they should grade as close as possible to what they normally would come fall. School officials hoped to send a message: Students must take virtual school seriously.

The apparent consequences of that decision, demonstrated afresh by the Fairfax statistics published this week, are confirming fears about how the pandemic is driving an equity gap in American education that may prove impossible to close. Fairfax’s data shows that children who are engaged and care deeply about school — children in stable home situations, whose parents have sufficient resources — will stay engaged in an online environment, while children whose temperament, socioeconomic status or home situation have historically barred them from academic achievement will slip further and further behind.

Children who were middling or poor students suddenly began earning more failing marks, including in classes they had not failed before, according to the Fairfax analysis. Historically low-performing students are seeing an explosion of C’s, D’s and F’s this semester, far more than would have been expected based on their pattern of achievement in past years.

“Results indicate a widening gap between students who were previously performing satisfactorily and those performing unsatisfactorily,” the report concludes. “Students who performed well previously primarily performed slightly better than expected during Q1 of this year.”

“In contrast, students who were previously not performing well, performed considerably less well,” it continues.

Even in normal times, factors such as socioeconomic status, race and whether parents speak English accounted for roughly two-thirds of student achievement scores and standardized test prowess, said Jack Schneider, an assistant professor of education at the University of Massachusetts at Lowell who directs research for the Massachusetts Consortium for Innovative Education and recently published a book on public-school testing.

At this phase of the pandemic, he said, the United States has reached a tipping point: The damage done to schoolchildren with scarce resources is likely to be irreparable. The best thing the nation can do would be to offer everyone a “do-over,” Schneider said.

“The default should be, once we’re in-person again, everybody could go back to the grade they were in March of 2020,” he said. “We need to slow the pace down in the name of equity.”

In Fairfax, whose 186,000 students make it the largest school system in Virginia, Superintendent Scott Brabrand said officials are working swiftly to boost grades. He noted many children who were performing well academically before the pandemic are still earning high marks, although he acknowledged that others “who previously struggled in school . . . continue to do so.”

Brabrand added in a statement: “We are working on identifying these students by name and by need and are working on specific interventions to support them right now and as we phase back in person.”

Fairfax returned several thousand students to school buildings over the course of the fall, prioritizing students with disabilities, those whose primary language is not English and prekindergarten through elementary students. But as cases rose in the Washington area, the superintendent this month halted plans for further returns and sent some groups back to all virtual learning.

On Monday, Brabrand announced that nearly 3,000 more students — elementary-schoolers and high school students taking career and technical classes — would return to online-only instruction.

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He said Fairfax has already tried to help struggling children by instituting “catch-up days” and extending the first-quarter grading period. The school system also revised its student workload to make it less onerous this semester, for example instructing teachers to give students no more than one hour per week of homework for each course.

And the school system adjusted grading: Fairfax teachers are supposed to provide “additional flexibility” on deadlines, to accommodate student absences without penalizing them and to allow ample opportunities for test retakes, according to guidance posted online.

But one Fairfax high school teacher, who spoke on the condition of anonymity for fear of retribution from the school system, said he is doing all of these things — and still, 50 to 70 percent of his 150 students are achieving D’s and F’s, whereas before they had earned B’s and C’s.

This teacher is pursuing a very generous late policy, he said. And whenever he finds out that a student is facing extreme circumstances at home — for example, the student who was evicted from his home midway through the pandemic, or the other student whose father recently got the coronavirus — he sits down with the child to develop an adjusted work schedule.

The problem is that students do not always tell him of their troubles. And, like his colleagues, he teaches far too many children to be able to sit down individually with every one of them to investigate their home life and then plan an individualized course of study. Nor can he relax standards entirely, because then he wouldn’t be doing his job as a teacher and making sure children learn.

“I’m working multiple extra hours per week to figure out ways to bridge the gap and get the kids where they need to be,” he said. “It really is exhausting. Co-workers of mine, usually some of the most energetic and vivacious, are worn down, too.”

Bob Farrace, spokesman for the National Association of Secondary School Principals, said educators nationwide are undergoing similar trials — and pushing themselves to the brink of breakdown to keep students learning and achieving in the classroom.

“But with so many challenges at once,” he said, “it’s like they’re holding back a tidal wave with a broom.”

Farrace said he is hearing about spikes in D’s and F’s from superintendents across the country. The Fairfax report, compiled over the past few weeks by the school system’s research arm, offers a window into how the consequences of online learning differ by things children have no control over: race, learning ability or whether they were born into English-speaking households.

Younger Fairfax students are struggling more than older ones: The percentage of middle-schoolers receiving at least two F’s quadrupled, while the percentage of high-schoolers scoring at least two F’s increased by 50 percent. The percentage of students with disabilities earning at least two F’s, meanwhile, more than doubled, while the percentage of children for whom English is a second language receiving at least two F’s rose by 106 percent to account for 35 percent of all children in this group.

Among racial groups, Hispanic students were most affected: The percentage of these students with at least two F’s jumped from 13 to 25 percent.

Comparing grades achieved in past years with grades this year showed that the drop in passing grades is significant and unprecedented. The likelihood of passing an English class decreased by 40 percent this year for all students, according to the analysis, while the likelihood of passing mathematics decreased by 30 percent.

Student achievement is seriously off track in these two subjects from what would have been expected based on past performance. According to the analysis, 35 percent of all Fairfax students are underperforming in math, and 39 percent are underperforming in English.

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Again, the dip is especially severe among vulnerable children and those for whom English is not their first language. Students with disabilities and Hispanic students both saw large spikes in underperformance, compared with other demographic groups studied.

But by far the biggest drop came for learners whose primary language is not English: Forty-seven percent are underperforming in math this year, while 53 percent are underperforming in English.

Fairfax mother Rocio Portillo, 35, said she was saddened but not surprised by the report. That’s because of what happened this semester to her daughter, a 17-year-old high school junior.

The teen achieved mostly A’s and B’s pre-pandemic. She was on track to fulfill her dream of studying forensic psychology in a four-year college. That wish is shared by her mother, who never finished college.

Then online learning hit. The teen cried every day during the first week this semester because she finds Zoom school nearly impossible to follow: The format is confusing and some teachers talk too fast. Despite hours of studying every day, her grades have slipped to C’s and one F.

Recently, Portillo sat down with her daughter to suggest attending community college.

“What breaks me is that I know she’s trying, I know she is giving it her all,” Portillo said. “She knows that college is important. She wants to be somebody in life.”