More than one kick and more than one punch was thrown. They are already repeat offenders. Not only did one of them kick him unprovoked, but the others kicked him repeatedly when he was down. Each of those is a separate criminal attack.
It will be treated as a single incident. Is that right? Honestly, I’m not sure. It if was my school, I’d have some say in it, and their previous record would count for a lot. So would the opinion of the parents of the victim, and the victim himself.
But expelling them from that school wouldn’t accomplish much (expelling them from my school would because of its particular reputation as one of the best schools - and we’d also feel more pressure to do so than a Regional Catholic College). It would not make them any less likely to do this again.