The majority of secondary schools in England sent home at least one pupil because of coronavirus last week, Government figures show.
About 6% to 7% of state school pupils did not attend class for coronavirus-related reasons on October 22, according to the Department for Education (DfE) statistics.
Approximately 26% of schools, excluding those on half-term, said they had one or more pupils self-isolating due to potential contact with a Covid-19 case at school, compared to 21% the week before.
This is 55% of secondary schools and 20% of primary schools.
Schools on half-term last week have been excluded from the analysis to give as clear a picture as possible of attendance in schools that would be open.
Overall, school attendance dropped from 89% a week earlier to 86% on October 22, the data suggests.
About 82% of secondary school pupils were in classes last week compared with 86% the week before, while attendance in primary schools dropped from 92% to 90%.
The figures come after access to Government-provided laptops was cut for some schools after the DfE changed its allocation process to align orders with areas with more pupils self-isolating.
The notification on Friday was sent the day after a new legal duty on schools to provide immediate access to remote learning to pupils at home because of Covid-19 came into force.
Up to 18% of schools said they had 30 or more pupils self-isolating due to potential contact with a case of Covid-19 inside the school, the data suggests.
The majority of pupils (between 4.8-6%) absent from class last week were self-isolating due to potential contact with a case of coronavirus.
About 0.7% of pupils were absent as their school was closed for coronavirus-related reasons, 0.4% were off as they suspected they had Covid-19 and 0.1% were off after testing positive for the illness.
The DfE says the data is not directly comparable with previous weeks as October 22 figures represent 92% of state schools compared to all schools.
Geoff Barton, general secretary of the Association of School and College Leaders (ASCL), said: “This analysis paints a grim picture of the increasingly challenging situation facing schools with attendance falling amidst rising Covid infection rates.
“In this turbulent context, it is crucial that schools are able to provide disadvantaged pupils with laptops so they can work from home if they have to self-isolate and do not have access to these devices.
“However, it is very clear that the government has completely underestimated the number of laptops that are needed.”
Mr Barton added: “Our impression is that the Government has never fully grasped the scale of the challenge both in terms of the numbers of devices that are needed and over ensuring that families have the connectivity they require.
“It is very frustrating that progress has been so slow on this front despite the fact that it has been discussed for many months.”
A DfE spokeswoman said: “Over 99% of schools have been open every week since term began and millions of pupils were attending last week, benefiting from time with their friends and teachers.
“As we would expect, some pupils are self-isolating in line with public health advice but the average size of those groups is relatively small compared to the total number of pupils on roll.
“Remote education should be provided from the first full school day that a child has to remain at home to ensure they do not fall behind.”