Sir Keir Starmer is urging ministers to put children “at the front of the queue” for coronavirus tests.
The Labour leader warned of a “flood” of school closures unless pupils were able to get the tests they needed.
His call came amid reports that 350 schools in England and Wales were forced to close completely or to send children home last week following positive Covid-19 tests.
Sir Keir said it was essential that children whose schooling was disrupted by the lockdown did not lose out even more because a shortage of tests meant they were unable to return to the classroom.
“If the Prime Minister does not get a grip of the testing crisis, children will be robbed of an education. We are seeing a growing flood of schools closures,” he said.
“The testing regime is not working, nor does it appreciate the unique challenges many families are having to cope with.
“That is why I’m urging the Prime Minister, like our key workers, to put children at the front of the queue for testing. To this week give parents a cast iron guarantee that they can get their child a test within 24 hours and the result back 24 hours later.”
His comments came at the start of Labour Connected – a four-day online event in place of the party’s annual conference, which was cancelled because of the pandemic.
Sir Keir is expected to use his keynote speech on Tuesday to set out his vision for Britain after Covid-19 and to explain how he intends to lead Labour back to power.