OK, I've just had a 5 minute look at the surveillance reports. They can be found here: www.gov.uk/government/publications/national-covid-19-surveillance-reports
Started on 23rd April. These are the numbers of outbreaks of 'acute respiratory infections' in schools every week since the 23rd April. They also give a confirmed 'how many were confirmed COVID positive test results', but as has been pointed out in other threads here, the chances of these being anything other than COVID are low - time of year, fact that 2 or more children/adults in a 'bubble' had the same symptoms (takes 2 or more to be called an outbreak). More likely to be false negative test results.
Starting from week 18 (dated 23rd April), weekly cases were:
2
1
4
3
15
14
24
44
40
55
49
30
One week is mysteriously missing around week 28/29, but might be that this was me doing a v quick 'open, find data, close' type search, so missed something, sorry.
Can you see where schools went back? To bubbles of 15 with extensive safety measures in place? Week 21 was half term, when lots of schools closed completely for the first time since Feb - 3 outbreaks. The following week was the magic 1st June. 15 outbreaks.