However, you do seem quite certain that infections are spreading easily through crowded classrooms. I agree.
Yes I do agree that infections can spread quite easily in classrooms.
Social distancing where just one infected, symptomatic pupil or staff member is breathing in and out in an enclosed, poorly ventilated space for long time periods, is pretty useless.
What measures should be taken to ensure that doesn't happen again when schools reopen?
For the billionth time, not social distancing - it's pointless in the circumstances (see above) and so therefore not worth the damaging social aspects to kids.
Also, not masks. They do very little, and therefore the little that they do is not worth the harms.
What you're relying on is the fact that most kids are asymptomatic, so few will spread it home, and the fact that most parents with kids of school age will not be seriously ill if they do.
If you're vulnerable, thinking that your kid will be safe because there's masks and social distancing is just fantasy. What you're relying on is a risk assessment of how likely your kid is to be asymptomatic vs your likelihood of dying if they and and they give it to you.
Masks and social distancing when they're in school will not be enough of a factor to change those odds.
It's either kid-at-home or kid-at-school decision time for those people.
As I've said - I think that people should be able to keep their kids at home without losing their school place until infection levels come down/the adult is vaccinated if this is the case.
All the other kids should be back at school asap, because for them, the risks to them and their parents from the virus lesser than the harms that occur to kids by not being in school.
Anyone under 50 with no serious co-morbidities or caring responsibilities for someone who has, who thinks that it's better for the children to be off school, is making a big mistake in their risk assessment, for which their children will suffer.
boring old mainstream WHO and SAGE
If you don't know about the mountains of errors that both the WHO and SAGE have made, then you've not been paying attention.