Does it need to be a track suit or would something like jazz-pants and a hoody do? That would be a lot more flattering and as long as the legs of the jazz-pants aren't too long (or they will pick up mud), it should be just as practical.
Tk Maxx often has good name sportswear pretty cheap. Second the advice to buy nice track pants/ joggers/ yoga pants and then get a nice sweatshirt. Joules and Boden very primary friendly and wash v well. Don't think you'll need a sports bra unless you have amazing norks! You will probably find you don't take it off- unless you are in y6 primary teachers spend a lot of time on the floor, climbing on desks, etc!