Yes and no. A citizen is guaranteed first amendment rights, but that guarantee doesn't allow a person to violate another person's first amendment rights.The superintendent claims that the cheerleaders violated the 1st Amendment. Not the sharpest knife in the drawer. If she doesn't know that the Constitution is binding on the government, not on citizens, I'm not sure she should be educating our children.
Anyhoo, to this topic, the cheerleaders weren't acting as individuals, they were acting as representatives of the school at the time of the incident. So the Amendment I restriction on government applies to them. Even if they weren't acting as representatives of the school per se, they still don't have Amendment I rights unbridled. For example, a profanity is protected speech, but if the school has a "no profanity" policy on its students, it's not a violation of Amendment I.