Fears over swine flu saw dozens of parents across England keeping their healthy children out of school during the last week of term.
The decision suggests some families may continue to withdraw pupils from class at the start of the new term to avoid infection, even if the Government decides schools should remain open.
The H1N1 virus is spreading rapidly, with about 1,000 schools in England reporting cases before the end of term.
Since last month, the Government's advice has been that containment is no longer possible and schools where pupils have the virus do not have to close.
The Department for Children, Schools and Families has updated its guidance to schools, saying the summer break may be extended, although it currently expects schools to re-open as normal in September.
But it will be monitoring developments over the holiday and contacting schools in the last week of August.
Mick Brookes, general secretary of heads union NAHT said the virus put his members in a difficult situation.
"The decision would have to be made in the balance of risks," he said. "I have heard of parents keeping their children off because of swine flu, but also heads saying they will shut the school and parents saying, `What do we do for childcare?'
"If a child has underlying health problems, it may be it is reasonable for parents to keep them off. The question is, (for) how long?
Teachers from across England told The TES that parents withdrew healthy children during the final week of term.
A teacher at a Leeds secondary where a teenager has been diagnosed with the virus said only half of Year 10 pupils had attended one of the final days of term.
"Loads of parents have been turning up to take their kids home since they found out we had swine flu," she said. "They don't want to increase the risk of their child catching flu and having to cancel their summer holidays. They might as well just shut the school."
Elsewhere, a primary teacher said one of her colleagues had contracted swine flu, as well as a nursery pupil, while other cases were suspected.
"We gave them the choice whether or not to still bring children, so on Monday and Tuesday I had a class of seven," she said.
"In fairness, it's the end of term and we are doing nothing productive. And I would be concerned if I had young children with asthma or other complications."
Over 100 British children and teachers on a trip to China were quarantined in their hotel last weekend after eight were diagnosed with swine flu (see picture, top). They were part of a tour organised by the Specialist Schools and Academies Trust and the British Council.
Of the total number of children affected in the UK, at least 652 are in hospital, including 53 in critical care.
Researchers at Imperial College London this week argued that there was a strong argument for school closures. Writing in The Lancet Infectious Diseases journal, they said early and prolonged closures could reduce the number of cases at the peak of the pandemic by up to 40 per cent.
But they also found that, in some cases, school closures had no effect, either because they came too late or because children mixed with each other outside school.
They also pointed out that school closure results in economic and social costs. Previous research on pandemics estimated that peak absence from the healthcare workforce at 45 per cent, with 30 per cent due to school closure.
Editorial, page 2
OPEN AND SHUT CASE: DO SCHOOL CLOSURES HALT SPREAD OF INFECTION?
2008: Hong Kong
Primary schools and kindergartens were closed for two weeks after the deaths of two children, apparently from influenza. A reduction in the number of cases followed, but this was expected as the outbreak had already peaked. The reduction was no greater than during a previous outbreak in 2007, when schools remained open.
Three weeks into the outbreak, elementary schools closed because of a teachers' strike; there was a 43 per cent reduction in weekly infections. After the strike, respiratory illness rates rebounded, suggesting the closures had affected the spread.
A recent study of the effect of holidays on influenza-like illnesses found that they prevent about one in six seasonal cases. But the authors pointed out that children's contact with their peers was sometimes reduced during holidays because they went abroad - something they might not do if closures took place in term-time.
Because of worries that school closures might increase anxiety and create a crisis in this flu outbreak, they were carried out piecemeal in different areas. Decisions to close individual schools were delayed, often until after 50-75 per cent of children had been ill. This late intervention was judged to be ineffective.
Conclusion: No, too late
School closure was just one of several measures - shutting churches, bans on mass gatherings, mandatory mask wearing and disinfection - taken to prevent the spread of the influenza virus. Mortality was reduced by 10 per cent to 30 per cent, but it is not possible to estimate the specific effect of school closure.
Source: The Lancet Infectious Diseases.