The thing proper

It is grammatically correct but does not mean mathematical schools. It means mathematics in the most strict, literal sense.

If by major mathematical schools you mean major mathematical philosophies or major groups of people who share a mathematical philosophy (such as those listed at Wikipedia), then one alternative would be:

… and this is before we consider major mathematical schools of thought.

There is no objection. Proper is one of the postpositive adjectives. Others are seen in phrases such as heir apparent, devil incarnate, body politic. See Wikipedia's page for more examples: Postpositive adjectives