School leaders have urged ministers to immediately publish data on the number of Covid-19 variant cases being found in schools and colleges in England, as outbreaks force a growing number of schools to close or send children home.

The leaders of eight trade unions representing school staff, including the four main teaching unions, have written to the education secretary, Gavin Williamson, asking for the data held by the government and Public Health England to be released.

The joint letter says: “There are growing concerns around the variant B.1.617.2 and reports from areas such as Bolton that cases are growing fastest amongst school-age children, with cases in Bolton higher now than at any point during the pandemic.”

The group said it urgently wanted to know when PHE first shared data with ministers on the spread of the B.1.617.2 variant within schools and colleges. “Did the government instruct PHE not to release this data? If so, why?” the letter asks Williamson.

The Department for Education’s latest attendance figures for England showed a sharp rise in absences related to Covid-19. Confirmed Covid cases among pupils rose by 1,000 to 4,000, double the level since schools returned after Easter. More than 60,000 pupils were self-isolating because of contacts within their schools, compared with 22,000 self-isolating because of contacts outside schools.

Although the number of school closures remains small, in the past week the numbers appear to have increased, with some schools deciding to switch to remote learning for the final week before the half-term holidays in most places next week.

On Tuesday, Haslingden high school in Lancashire closed its site to most pupils until after the half-term break, with the headteacher, Mark Jackson, telling parents: “In the thousands of tests conducted up until last weekend, we had only had one positive case. Over the last week, we have seen the number of positive cases increase considerably and it is clear that the virus is being transmitted between students in school.”

In Bolton, which has recorded a rapid increase in cases involving school-age children, public health authorities have set up “surge testing” within several schools this week to identify new cases more quickly, although the local authority insists the new coronavirus variant is mainly spreading through contacts outside schools.

In Gloucestershire, Abbeymead primary school has closed after an outbreak that began last week through a member of the National Tutoring Programme, part of the government’s catchup learning scheme.

Data published by the Office for National Statistics on Thursday found that the percentage of school staff testing positive for Covid antibodies resulting from infection “increased significantly” between December and March, when schools fully reopened.

Overall, one in five school staff in the areas surveyed were found to have antibodies acquired through infection, with staff working in Liverpool, Lancashire, and Barking and Dagenham local authorities having the highest rates. In primary schools the rate increased from 15% in December to 25% in March.

The ONS survey, based on work it carried out with PHE and the London School of Hygiene & Tropical Medicine, also found that secondary school pupils were more likely to have contracted Covid-19 than those in primaries – in December, 13.5% of secondary pupils had antibodies, compared with 9% of those in primary schools.