Schools were scores of languages are spoken are common, as are classrooms where English is not in the majority. I don't think this helps kids whose mothertongue is English and I don't think it helps those whose whose second language is English. Everybody needs to be on the same page in the classroom and the faster everyone is up to the speed the better. All learning is hampered until everyone speaks/writes/reads one language to a fluent level, so it's worth concentrating wholly on getting that bit right before wasting time teaching people stuff they aren't able to understand and therefore perform to their full capacity.
Fast-track English lessons so that all children whose first language is not English can speak, read and write it to the level of nationals in their same age group.