Around one in six state secondary schools in England were not fully open last week, latest government figures have show.
Eighty-four per cent were open to all students last Thursday, according to Department for Education (DfE) data.
This is a drop from 92 per cent on 17 September.
Partial closures were mainly due to coronavirus-related reasons, the DfE said.
A tiny proportion of state schools – 0.2 per cent – were completely closed on 24 September, according to the latest available figures.
Meanwhile, 93 per cent of state schools in total were fully open - down from 94 per cent the week before.
The latest figures come amid a surge in coronavirus cases in the UK, which has reported some of its highest ever daily tallies in recent days.
Six per cent of state schools who responded to the government survey said they were not fully open due to a suspected or confirmed coronavirus case last Thursday.
This equates to around 1,000 schools in England.
Commenting on the latest figures, Geoff Barton, General Secretary of the Association of School and College Leaders, said: “We are extremely concerned to see a drop in the number of secondary schools fully open due to Covid cases and the resulting requirement for groups of pupils to self-isolate."
"This reflects the extremely difficult circumstances in which schools are operating amidst rising infection rates in the community."
Employees were also told to work from home again where possible.
Gavin Williamson, the education secretary, said: “It is encouraging to see that the vast majority of schools remain open, with more than 7.2 million pupils estimated to be attending schools last week gaining all the benefits of being in the classroom.
“Only a small minority of pupils are currently self-isolating and there is remote education provision in place for the short time they are unable to attend school."
He added: “I’m hugely reassured schools have the right protective measures in place to reduce the spread of the virus, and are only asking close contacts of confirmed cases to self-isolate."