I’m sorry I haven’t read all of the thread.
DH is a teacher, I work admin in the same (massive, oversubscribed) school here in France.
I cannot understand why schools in the U.K. haven’t made masks mandatory.
Here, every single person - teachers, students from 6 yrs up, TAs, admin, cleaners, management - wear a mask all the time. Even the more nervous teachers have accepted that the school is not a hotbed of infection. Over 2000 students and 100 teachers, and we haven’t had a whole class closure since the very start of the year.
Masks are the only difference I can see btw us and you, and less stringent “contact case” / isolation rules. We have exactly the same issue of small classrooms, careless teens etc, but the difference is that our protocol says “distancing is best, but where not possible masks must be worn by everyone, all the time”. Since distancing is impossible this has translated into “wear a mask - everyone and always”.