LONDON/WASHINGTON — Millions of British students have returned to school following the Christmas and New Year holidays, amid a record surge in infections and hospitalizations. For teachers and parents, the situation has brought a grim sense of déjà vu.

Unlike last January, when the rampaging Alpha variant plunged the United Kingdom into another lockdown, Prime Minister Boris Johnson has decided to "ride out" the Omicron wave with limited restrictions and to keep schools open, citing the toll remote education has taken on students' mental health and learning.

But leading teachers' unions say the government hasn't done enough to keep classrooms safe, demanding financial support for air-cleaning units, on-site Covid-19 testing and substitute teaching staff in a rare joint letter.

Ahead of the new term, the Department for Education announced twice-weekly testing requirements and temporary masking for students in middle and high schools, but not for elementary school students. The department said it would be providing 7,000 air-cleaning units for teaching spaces where quick fixes, like opening windows, are impossible -- approximately one for every three schools in England. To plug staffing gaps, the UK's education minister has also suggested combining classes, and called on retired teachers to show "Blitz spirit" to return to the classroom.

"It's woefully inadequate," said Stuart Guest, a head teacher at an elementary school in Birmingham, England, of the government's measures. "They keep saying education is their number one priority. It's clear it absolutely isn't." Last Wednesday, five of his staff, including three of his 15 classroom teachers, were out sick or isolating -- the most absent from Guest's school since the start of the pandemic -- and he said he feared more would follow.

As Omicron takes its toll around the world, the ramshackle infrastructure that has kept schools running over the past year is in jeopardy. The variant, which has spurred a record rise in pediatric infections in the United Kingdom, parts of Europe and the United States, threatens to upend the shaky balance that allowed schools to mostly stay open last year -- albeit with targeted classroom closures.

In the US, more children are being admitted to hospitals than ever before. The Biden administration has said schools are "more than equipped" to stay open, as Omicron rips across the country. But some elected officials are erring on the side of caution by delaying the new term, while a teachers' union has forced public schools in Chicago, Illinois, to shutter amid criticism from members that conditions in classrooms are dangerous.

Though outbreaks in schools can occur, studies in the UK, Australia, US and Italy have shown that spread among children in school settings is typically lower than -- or at least similar to -- levels of community transmission, when mitigation strategies like masks are in place. But experts are waiting to see whether the more contagious Omicron variant may change that calculus, and trigger an explosion in cases in classrooms.

While children have been less likely than adults to develop severe illness from Covid-19, they can still get very sick or die, or they could develop complications like life-threatening multisystem inflammatory syndrome and long Covid. Up to one in seven children and young people may have Covid-19 symptoms as many as 15 weeks after illness, a study by the UCL Great Ormond Street Hospital in central London found.

According to the UK Office of National Statistics (ONS), 117,000 children in the UK are now living with long Covid.

Omicron appears to be causing milder illness than previous variants, but early research has also suggested that Omicron may trigger more upper-airway problems, which can be more dangerous for young children, potentially leading to croup and bronchiolitis.

"This narrative that it's just a mild virus is not accurate," Dr. Peter Hotez, co-director for vaccine development at Texas Children's Hospital, told CNN's Jake Tapper last week, remarking on the surge in Covid-19 hospitalizations at America's largest pediatric hospital.

About 17% of US children age 5 to 11 are fully vaccinated; the vast majority of children in hospital in the US are unvaccinated, according to CDC director Rochelle Walensky.

Two years into the pandemic, public health experts have made clear which mitigation measures would make classrooms safer: vaccination, masks in classrooms, regular testing, contact tracing, bubble-systems, isolation and improved ventilation.

The World Health Organization's regional director for Europe, Dr. Hans Kluge, said in early December that masks and ventilation, combined with testing, should be a standard in all primary schools and that vaccinating children should be considered nationally.

Many EU countries began vaccinating 5- to 11-year-olds in mid-December, but the UK has only recently started rolling out shots to clinically vulnerable children. That has vexed some parents, with British news reports of at least one mother driving to Europe to get her daughter vaccinated.

Christina Pagel, the director of the Clinical Operational Research Unit at University College London, said the decision to delay shots for British children was a mistake, one that will set them up for another term of illness, and would put parents and school staff at further risk. Going into the new term, one in 10 children (aged 2 to 11) had Covid-19 in England, compared with one in 15 adults, according to ONS data.

"We're going to see a big surge in kids. And it's going to be very, very disruptive," Pagel said. "Their teachers are going to get it, and probably in about a month or two, their parents are going to get it. We're going to see a prolonged high level of infection."

Pagel, a member of Independent SAGE, a group of expert scientific advisers unaffiliated with the government, said the British government has been an outlier in its attitude to infections in children and has failed to use mitigation strategies deployed successfully elsewhere -- crucially, ventilation and masks.

In an op-ed for the British Medical Journal, she highlighted the successes of countries like Norway and Japan, which have kept schools open through preventative measures, like spacing out classroom desks, using outdoor spaces as much as possible, avoiding crowding and using rigorous hand-washing practices. Pagel also noted that parts of the US and Europe, including France, Spain, Italy and Germany, have successfully mandated masks for elementary, middle and high school children, with surges in cases when rules were relaxed.

A recent study in Germany found that requiring children and adults to wear face masks in schools could significantly reduce Covid-19 transmissions and outbreaks, because clusters were generally more severe when an adult was the source. Studies released by the US Centers for Disease Control and Prevention (CDC) have also shown the efficacy of masks in limiting Covid-19 transmission in education settings.

But even in countries like France, where masks and other mitigative measures are mandated in schools, there are fears that Omicron will run amok in classrooms. A December analysis of protocols in French schools, which used data from the country's spring wave of infections, found that regular testing and vaccination against Covid-19 were the most effective ways to minimize closures. It also suggests that viral circulation among children in schools is much higher than in the general community, resulting in greater risk of infection for students' household members.

Vittoria Colizza, a Paris-based epidemiologist at Inserm, the French public-health research center, and one of the authors of the study, said that in light of Omicron and the threat of even more contagious variants, the situation in schools was set to become more difficult in the coming months. "I think much of the problem has been this biased narrative that transmission in schools doesn't happen, but of course it happens," she said.

As students return to school amid Omicron, many scientists and public health experts are asking: If keeping schools open is so important for children, then why are we skimping on measures that could protect them?

"It's quite terrifying looking at the number of children who are being infected and hospitalized every week. And the fact that they are going into schools now with a highly transmissible variant with almost no mitigations and with the vast majority unvaccinated," Dr. Deepti Gurdasani, a clinical epidemiologist at Queen Mary University of London, said of the situation in England. "The rhetoric still very much seems to be to keep schools open at all costs ... rather than the real discussion we should be having, which is how do we keep schools open safely.

"I'm not even sure many parents realize the sort of risks that children are facing every day. And there are countries that have done so much better than us," she added, noting successes in Southeast Asia and Western Europe, particularly in curtailing airborne transmission.

The same strategies that have made Southeast and East Asia so successful at suppressing the virus -- widespread mask use, improved ventilation, comprehensive contact testing and tracing programs, and supported isolation -- have also allowed them to limit disruption to education, Gurdasani said, flagging public health measures in Japan, Singapore, South Korea, and Taiwan. They've also remained dynamic, shifting protective measures depending on cases, and quickly moving to hybrid learning. — CNN