The issue came from a student publicly punching another one on the bus and also putting the school into disrepute as members of the public witnessed and reported it to the school too. Parents of the victim went to the police and made an official complaint.
From a school perspective we ensured the two students were not in contact or leaving school at the same time. Sanctions were applied. The fact that the parents reported it to the police our SSO got involved.
The way it was dealt with was planned ahead and with respect of the school rules and protocols. A conversation recapping everything happened between myself and the SSO. The SSO contacted the parents in front of me first introducing themselves letting them know what happened (on top of the school doing it prior) and why they would need to speak to their child. Once the parents were aware, we spoke to the students separately and the difference that made to our students is how this time it came from a police officer and how what they have done was something that could be extremely dangerous and how it could have been dealt using the school's system instead. It kind of reinforced what the school consistently tells them and put a ""reality"" feel to it instead of just the teachers saying it.
The outcome was almost instantly observable as both did not get into any other altercation since then. One of them is even doing better in class in terms of behaviour and getting more merits. The success resides mainly in the excellent communication skills of the SSO and how they make an effort to work with the school and its policies.