Sports are not strictly banned, at least not explicitly so. They always harp on the "danger" of worldly association that comes with such sports, and of course that it takes away from field circus. How can you go out in field circus if you are at soccer practice or at the mathalon? Or, what if your mathalon happens to be on an evening where there is a "Family Worship" or a boasting session? (And note, mathalons are by nature non-violent competitions where teams take difficult math quizzes and get points based on correct answers).
You will not get disfellowshipped for playing sports (except those deemed violent). However, you will receive the houndings that your boasting session attendance and field circus activity will suffer. Plus, the worldly association might cause you to think about whether you really are in the truth or not.