More than four in five schools in England have children absent from class because they cannot access a Covid-19 test, according to a survey.
The majority of schools (94%) have pupils staying at home due to suspected or confirmed cases of coronavirus this term.
More than three in four (78%) have had staff having to self-isolate, according to a poll by the school leaders union NAHT.
Meanwhile, nearly nine in 10 schools (87%) have children who are absent because they are waiting for test results.
The results suggest 82% have pupils not in class because they cannot access a test.
School heads and governors, including the NAHT, have implored Boris Johnson to “take charge” of tackling the testing delays to ensure schools remain open.
Paul Whiteman, general secretary of the NAHT, has warned children’s education is being “needlessly disrupted” by a testing system which is in “chaos”.
Mr Whiteman said: “Tests for Covid-19 need to be readily available for everyone so that pupils and staff who get negative results can get back into school quickly.
“But we are hearing the same thing repeatedly from our members across the country – chaos is being caused by the inability of staff and families to successfully get tested when they display symptoms.
“This means schools are struggling with staffing, having to send groups of students home to isolate or close classes, and ultimately that children’s education is being needlessly disrupted.”
One in seven schools (14%) have had confirmed cases of Covid-19 since they began welcoming back students for the autumn term, the poll suggests.
The survey, that took in 736 school leaders over 24 hours, found that three in five (60%) have staff staying home because they are waiting for test results.
Mr Whiteman added: “It is in no way unpredictable or surprising that the demand for Covid-19 tests would spike when schools reopened more widely this term. And yet the system is in chaos."
A government spokesperson said: “The latest official statistics show that 99.9% of schools are open with the vast majority of pupils attending.
“Where staff or children have symptoms of Covid-19, testing capacity is the highest it has ever been, and we are working to provide further priority access for teachers.
“Schools only need to identify close contacts and ask them to self-isolate if and when a case is confirmed from a positive test result.
“Close contacts of confirmed cases must follow the full 14-day self-isolation period and should only seek a test if they have symptoms.”