For the trip home on the bus, he could keep an eye open to see if that group are getting in the bus, let them get on, then wait for the next one.
In the playground, there is probably just the one ringleader. He could try simply ignoring them ... walk away from them whenever they start. If they follow him, walk away again. Do this a few times, then when it starts once more, confront the ringleader and ask him/her why, if he is that offensive to look at, they insist on following him. You might need to help him with a form of words - the idea is to embarrass the ringleader in front of her/his mates (that trick worked for my daughter at primary school, but I can't recall what it was she said). A few years later, at a parents evening at the secondary school, I was talking to the ringleader's mother when the girl (seen by my daughter as her worst enemy) piped up and told me that she really admired my lass for her ability to ignore the chaos around her in class and get her head down and learn. My girl was astounded when I mentioned it to her.
And talk to the school anyway, despite what he wants. There should be staff of some kind in the playground during breaks who can keep an eye on what is happening and intervene if needed, without giving away the fact that you have spoken to them.