The thing is, on the legal merits I'm inclined to think this is the right decision. A cheerleader acts as a mouthpiece for the school -- that's her job -- and it makes sense that she can be compelled to speak the message the school wants sent (cheering for the players).
Nonetheless, if I were on the Supreme Court, I'd be inclined to write an opinion concurring in the denial of cert, saying something like:
It is well-established that a denial of certiorari entails no judgment on the merits of the case. It should also be established that such a denial entails no judgment on the merits of the underlying conduct. The constitution permits people, as well as government agencies, to be stupid, foolish, irresponsible, insensitive, or flat-out jerks. The type of moral judgment that impels one to kick out a cheerleader because she refused to cheer on a man who was convicted of assaulting her is the sort that makes one shudder to think these officials are in charge of crafting any sort of educational policy whatsoever. Unfortunately, we lack the authority to order the officials to develop basic human empathy, and ultimately those sorts of failures often lie beyond the capacity of law to remedy.
Something like that, anyway.