The chance of false positives may be very low as ministers suggest.
But given that millions of schoolchildren are being offered a test, it could still lead to thousands of people having to self-isolate for no reason.
If just half of pupils take up the offer of three tests in school, and the false positive rate is 0.1%, that would lead to around 6,000 pupils being asked to isolate at home over the next week or so when they are not infected.
The rest of their family will also have to isolate, which means if they have siblings, they too will miss school.
What is more, if the positive comes from the second or third test then the individual’s close contacts in school will also be affected.
It means many thousands of children could be wrongly denied the chance of being in school after already spending the past two months at home.
But what is particularly baffling experts is that it is so unnecessary.
Getting the test confirmed by a PCR test processed by a lab would solve the problem.
By sticking to their guns, ministers could end up undermining the whole initiative.
Covid: PM leading No 10 briefing as England schools return – BBC News