I may also explain to them that if defending oneself receives the exact same penalty that attacking someone gets it will usually be best to initiate the combat yourself.

This is excellent advice, with the caveat that the school's disciplinary penalty is probably not the only cost. Being known as "the kid who walks expressionlessly up to other kids and punches them in the testicles without warning" may be a significant penalty too. (This doesn't mean striking first is always a bad strategy, just that it needs to be done carefully).