I'd be really interested to know whether infections in primary are less likely to spread once they are in the class. Because anecdotally in my area, there's been a lot of primaries that have just had staff cases identified and it hasn't spread between staff, but then there have been others where pupils have tested positive and that bubble has been in and out like a yoyo.
My hunch is that bubble size plays a large role, as does the fact that primary pupils travel less to get to school and are more closely supervised out of school. I'm not entirely convinced that once it is within the pupils in a bubble that it spreads significantly less than in other settings, but that at least the fact that most schools do bubble closure instead of close contacts does mitigate this further.