Every female swimmer who is up against Thomas in a swimming event should refuse to compete. Let Thomas “win”, alone in the pool. After a few times that this happens, the idiots who run these events will have to change course.
The problem is that they won’t commit to do what is right, ie boycott. All the parents should boycott the events too. Let the school lose money on the event. Like you said, all it woild take is a few times, but even if it took a year or more, at least they would be fighting back for something that is far more valuable than a trophy- moral decency!