The video evidence will probably be submissable in court, because it was not a hidden camera. Since people could see that it was being filmed and no one objected, it's reasonable to assume that you would be aware of the video tapes existance and that it would be cataloging the event. However, there is no way the kid would win the suit, or that the video would have any sway in it. A judge can't watch a video and be like, 'oh yeah, he IS a really good drummer'. That is completely objective and it is a situation where people are being selected based on drum merit, the people in charge of the selection are the ones best entitled to make that decision, not the kids mom or a judge. The kid would need witnesses to testify that he is a great drummer who also has some established knowledge in the field, or would need to have a judge on tape saying 'boy, he's really good, but since he's a retard, he can't be on the squad, 3 cheers for discrimination!'. You really can't counter sue for anything though, because you have not encurred any damage to your reputation or finances. The only resonable countersuit would be for court costs, anything else the judge will toss out.