Will Grogan stared blankly at his biology classwork. It was material he had mastered the day before, but it looked utterly unfamiliar.

“I don’t know what you’re talking about,” he blurted. His teacher and classmates reminded him how adeptly he’d answered questions about the topic during the previous class. “I’ve never seen this before,” he insisted, becoming so distressed that the teacher excused him to visit the school nurse.

The episode, earlier this year, was one of numerous cognitive mix-ups that plagued 15-year-old Will, after he contracted the coronavirus in October, along with issues like fatigue and severe leg pain.

As young people prepare to return to school in many places across the world, some are struggling to recover from lingering post-covid neurological, physical or psychiatric symptoms. Often called “long covid,” the symptoms and their duration vary, as does the severity.

Studies estimate long covid may affect between 10 per cent and 30 per cent of adults infected with the coronavirus. Estimates from the handful of studies of children so far range widely. In April, Dr Francis Collins, director of the National Institutes of Health in America, cited one study suggesting that between 11 per cent and 15 per cent of infected youths might “end up with this long-term consequence, which can be pretty devastating in terms of things like school performance”.

Brain fog caused him to see “numbers floating off the page” in math

Doctors say even youths with mild or asymptomatic initial infections may experience long covid – confounding, sometimes debilitating issues that disrupt their schooling, sleep, extracurricular activities and other aspects of life. “The potential impact is huge,” said Dr Avindra Nath, chief of infections of the nervous system at the National Institute of Neurological Disorders and Stroke in the US. “I mean, they’re in their formative years. Once you start falling behind, it’s very hard because the kids lose their own self-confidence too. It’s a downward spiral.”

Will, a scout, a talented tennis player and a highly motivated student who loves studying languages so much that he takes both French and Arabic, said he used to feel “taking naps is a waste of sunlight.”

But covid made him so fatigued that he could barely leave his bed for 35 days, and he was so dizzy that he had to sit to keep from fainting in the shower. When he returned to his classes, brain fog caused him to see “numbers floating off the page” in math, to forget to turn in a history paper on Japanese Samurai he’d written days earlier and to insert fragments of French into an English assignment.

“I handed it to my teacher, and she was like ‘Will, is this your scratch notes?’” said Will, adding that he worried: “Am I going to be able to be a good student ever again? Because this is really scary.”

At Boston Children’s Hospital, where a program draws long covid patients from across the country, “we’re seeing things like fatigue, headaches, brain fog, memory and concentration difficulties, sleep disturbances, ongoing change in smell and taste,” said Dr Molly Wilson-Murphy, a neuroinfectious diseases specialist there. She said most patients were “kids who had covid and weren’t hospitalised, recovered at home, and then they have symptoms that just never go away – or they seem to get totally better and then a couple of weeks or a month or so after, they develop symptoms.

“We don’t yet have any sort of good predictors of who will be affected, how much they’ll be affected and how quickly they’ll recover. We don’t have any sort of magic treatment.”

Much about long covid remains mysterious. Some symptoms resemble aftereffects of concussions and other brain injuries. Some, such as post-exertional malaise – when physical or mental exertion increases exhaustion – echo symptoms of chronic fatigue syndrome, experts say.

Some patients develop Postural Orthostatic Tachycardia Syndrome, or POTS, which involves lightheadedness and racing heart rates upon standing up. Some studies report higher proportions of older children with long-term issues. That might be because adolescents find some symptoms more disruptive or because after puberty, hormones might amplify immune responses, Nath said.

An April study by the UK’s Office for National Statistics found that 9.8 per cent of 2-11-year-olds and 13 per cent of 12-16-year-olds infected with the coronavirus reported continuing symptoms five weeks later. After 12 weeks, rates remained significant – 7.4 per cent in the younger group and 8.2 per cent in the older group.