I'm not teacher bashing here but I'm pretty sure this is just luck rather than hard work on behalf of schools.
Yep. Lots of people on here reporting that their schools were fine, no cases or very few cases, and then WHAM, all the cases.
It's luck. There's nothing that schools can do to stop an airborne virus spreading around an indoor environment where teens are packed in like sardines and not wearing masks (per DfE guidance) except cross their fingers and hope that it doesn't get in.