Nearly a fifth of secondary schools in England sent at least some pupils home last week to isolate, amid concerns about Covid-19 cases.
However, official statistics published by the Department for Education suggested that groups of pupils were being sent home in smaller numbers.
Overall, in secondaries, which have had more virus cases than primaries, the attendance rate was 86% - meaning 490,000 pupils missed some school.
In primaries, attendance was 93%.
The figures, which are being published weekly by the DfE, showed 82% of secondary schools were fully open.
‘Lack of support’
This means 18% sent groups of pupils home because of public health guidance to minimise the spread of Covid-19.
There are no details of how many pupils this affected or where the schools involved are.
Geoff Barton, General Secretary of the Association of School and College Leaders, said: “This is the second week of a downward trend in the number of fully open secondary schools, and means that since 17 September the number that are fully open has fallen by 10% from 92% to 82%.
“It reflects the extremely tough circumstances in which schools are operating due to the impact of Covid.
“We remain concerned that schools lack the support from the government that they need in this challenging task.”
The statistical release said: “Although the proportion of fully open secondary schools decreased from 84% to 82%, attendance increased among both fully open and not fully open secondary schools.
“This suggests that, where groups of pupils are being asked to self-isolate, they are becoming smaller.
“Overall, attendance in state-funded secondaries increased from approximately 84% to 86%.”
It added that where schools are sending pupils home they are expected to be immediately offering remote learning.
Most schools have now been open for about a month since term started.
Teachers and head teachers have been complaining about difficulties in getting through to public health officials for advice and in accessing speedy testing.
Education Secretary Gavin Williamson said: “We will continue to work with schools to ensure all appropriate steps are taken to keep pupils and staff safe.”