Schools in England are being "severely hampered" by delays in Covid tests for teachers, says a head teachers' leader.
Jules White, organiser of the WorthLess? network of over 5,000 heads, says there is growing frustration at the lack of access to testing.
This means teachers have to isolate and that "serious staff shortages" could force partial closures in school.
But a government spokeswoman said "testing capacity is the highest it has ever been".
Mr White, a West Sussex head teacher whose group grew out of a funding campaign, has written to England's Education Secretary Gavin Williamson to say that efforts to get pupils back for the autumn term are being seriously undermined by a "test and trace system that is simply not working effectively enough".
Writing on behalf of a group of head teachers across 75 local authorities, he warns that schools are struggling to cope with teachers not being able to get quickly tested for Covid-19 and find out whether they can get back to the classroom.
The head teachers are reporting difficulties in getting tests and then delays in finding out the results - leaving staff in isolation and "out of action".
The letter from heads warns the government this problem is at a time when schools need to help pupils catch up and get ready for exams next year - and instead the lack of staff could mean even more lost lesson time.
Image copyright PA Media Image caption Pupils are back in schools but they face safety measures against the spread of Covid-19They also warn this uncertainty about Covid cases could be further compounded by seasonal "coughs and colds" - and that urgent action is needed on testing, rather than "vague promises".
"It is beyond frustration that we are now seeing teachers and support staff being unable to attend work because they cannot get a test or the results from it are far too slow," said Mr White.
"Time after time, schools are doing their utmost to support the national effort and time after time, we are left confounded by a lack of effective support from government."
Tehmina Hashmi, head of Bradford Academy in West Yorkshire, who is supporting the letter, warns of the confusion facing her school community over testing.
"It feels really tense in Bradford," she says, with parents reporting that they cannot get Covid tests.
Ms Hashmi says after working hard to get the school ready through the summer, there is great "frustration" at what she says has been "inept leadership" over Covid testing.
Last week Geoff Barton, leader of the ASCL head teachers' union, said teachers were struggling to get tests locally and were being directed to testing sites hundreds of miles away.
An academy trust leader has called for a more "robust strategy" to help schools facing Covid cases among pupils or staff - as more schools are going to face disruption.
Steve Chalke, chief executive of the Oasis academy trust, said that several of the trust's school have already had to send home year groups, affecting about 1,200 pupils.
He says that Covid testing needed to be available on the school site and results needed to be turned around quickly - and that eventually there would need to be routine, daily testing.
Mr Chalke argued that it would be better to accept the need for a planned rota system, with pupils switching between school and online learning at home, rather than having a "rotation system by default" each time a case was discovered.
He is also calling for a more "credible" approach to how next year's exam season will operate in a fair way and wants a big increase in the pupil premium, which provides schools with extra funding for disadvantaged pupils.
A government spokeswoman said that testing levels have increased - "but we are seeing a significant demand for tests. It is vital that children and school staff only get a test if they develop coronavirus symptoms".
"If a positive case is confirmed in a school, swift action is being taken to ask those who have been in close contact to self-isolate, and Public Health England's local health protection teams continue to support and advise schools in this situation," said the government spokeswoman.
"Children who are self-isolating will receive remote education. We will continue to work with schools to ensure all appropriate steps are taken to keep pupils and staff safe."