Rather than having a hard and fast rule about absolute numbers - that then has to be ignored due to the many exceptions - why not have a ratio system? Perhaps they already so but its not mentioned. Does that only kick in above 35?

What are head teachers supposed to do when faced with children in care or with SEN who need a space? You can't just magic up a new class room and split the class in two, or send settled pupils elsewhere! All they can do (which I know my local school did) is employ another teacher or class room assistant. At primary level they are often split into small groups anyway. Its still 1 class but 36 children now have 3 teachers/teaching assistants