I'm not an expert here but I'll have a go! Firstly my sympathies. The ability range is normal but over 30 in class makes life very hard for a teacher. As far a legal maximum class size is concerned for KS2 I don't think there is one. The school will have a PAN (Published Admission Number) this is the maximum number of children for the school. If a school has vacancies and a new pupil applies they have to be accepted unless they have been expelled from 2 other schools. I could be wrong on some of the above but basically you're stuffed! I once taught 42 in a year 5 class in the afternoons.