Because it is known that schools are one of the main vectors driving the spread. There are a number of issues.
Many primary children aren't tested (they often dont show the main 3 symptoms. But also parents are reluctant to test them.)
1 in 3 cases are asymptomatic so people are coming into school with covid and not knowing it. Our secondary did rapid flow testing for yrs 11-13 as part of a national test. 7 children in one tutor group tested positive meaning the year needed to be sent home! Imagine by extention how many families that would spread to -and then onto workplaces/shops as they spread. Yet this wouldn't have been picked up without the extra testing.
Just because your school might not have many cases that isn't the national picture. Also be aware that many schools are just not telling people beyond close contacts of the cases. I realised we were only being told those in our year group...
Have a look on here for some of Nobelgiraffes threads. She explains it very well. Think about what we know about the virus - we are told to wear a mask, stay 2m apart and avoid mixing indoors, especially in unventliated rooms. Schools are doing this with 30kids in a room and then mixing with other children, especially at secondary.
The graphs at the recent press conferences showed that the increase in cases was mainly school children. Again, they will be on Nobel's threads.
It really is quite obvious now. Ask any teacher...