Groups of pupils should only be sent home if there has been a positive test, rather than someone just developing symptoms, the prime minister has said.
Boris Johnson said it was important for teachers and parents to examine the Public Health England guidance about coronavirus testing.
Speaking to MPs on the Liaison Committee, he reiterated government policy when he said: "The reasons for sending such a class home, or a bubble home, would be if somebody tests positive."
It comes as a teaching union leader warned that schools are struggling to cope with a lack of COVID-19 tests for pupils and staff as the situation is becoming "increasingly out of control".
Dr Patrick Roach, general secretary of the NASUWT teaching union, has called on the government to prioritise the education sector for the allocation of tests in light of widespread challenges.
And the Association of School and College Leaders (ASCL) said it has received 264 responses from schools and colleges in recent days where leaders have said they have symptomatic staff and/or pupils who were struggling to access tests.
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Mr Johnson admitted overall demand fortests in England has "massively accelerated" in the last two weeks and there is not enough capacity to meet it.
He promised daily testing capacity would reach 500,000 by the end of October.
The PM insisted: "We have massively increased our testing capacity. I know that many people have had infuriating experiences, and I do sympathise with them.
"89% get their results within 24 hours, if you have an in-person test.
"And the distance that you have to travel to get a test has come down just in the last week. On average from about six or seven miles to about five miles."
The PM also told MPs that the government will "very shortly" be setting out "the priority of the groups that we think should have tests".
He added: "In schools, it's very important that parents and teachers should look at the guidance about when you should get a test."
Justice Secretary Robert Buckland has told Sky News' Kay Burley that schoolchildren and their parents could be prioritised for coronavirus tests – after hospitals and care homes – as the government deals with "real challenges" in the system.
The government has come under growing pressure over a lack of availability of tests in some parts of England – blamed on problems with laboratory capacity – and admitted it could take a Read More – Source