The girls themselves said they were seniors, and nothing untoward happened. Yes, if he were their teacher at the time, he probably should have been fired for it. That doesn’t make it illegal. Immoral? I don’t think so if he were single and in early twenties.
Two of the students said the two parties they saw him at were post-graduation. Most people are 18 by then. Aside from all that, we don’t know if these are even students, if other school personnel were there or if he’s at a friend’s or family’s party.