(CNN)As Covid-19 cases skyrocketed across Britain in late December, Stuart Guest spent his vacation poring over scientific reports about air cleaning and filtration systems.
Guest, a head teacher at an elementary school in Birmingham, England, scoured Amazon for affordable air purifiers in the hopes of stopping the more transmissible Omicron variant from spreading among his 460 students, who are between 3 and 11 years old.
The British government recommends two models made by Dyson and Camfil, but at £424 ($575) and £1,170 ($1,590), respectively, they were too expensive. Guest ultimately bought £200 ($270) portable units for each classroom.
"I got what I think is the best air purifier for the budget I have available. I hope I've got something that's doing the job, but I'm not an expert. And there's been no guidance put out by the Department for Education. I've had to do it all myself, and I shouldn't have to do that when it's a national crisis," Guest said.
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 Guest 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. It's also cast a harsh spotlight on just how little has been done in some places to protect students, with teachers resorting to propping open windows in freezing weather and frequently checking carbon dioxide monitors while delivering their lessons.
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 forced public schools in Chicago, Illinois, to shutter for a week 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.
"We've just done a terrible job vaccinating our kids across the country. ... So even though there's a lot of happy talk about the Omicron variant, less severe disease, when you add up all the factors ... we've got a very serious situation facing us in this country, especially for the kids."
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 f