The other advantage of having everything in the same school is that it makes moving up (and down) in the sets a lot easier. A kid who is right on the cusp between set 1 and set 2 in math can be put into set 1 for a term and see how it works out. Or a kid who, for whatever reason, starts slipping can be moved down a set and given some extra support. If these changes required moving schools then that will never happen.