I would double-check the laws in your area. For us, the school is responsible for the kids from the moment they leave their front gate through the day and until they reach home. Anything happening on the way home, you should be able to ask the school for support to keep your child safe. If a child misbehaves on the school bus (which is privately owned and paid for by the government) then the bus company proprietor notifies the school who then notify the parents. For our kids, their subsidised bus travel can be revoked for misbehaviour (loss of bus pass). For kids ineligible for free travel, they can be refused transport at the discretion of the bus driver. Again, the school has to be notified and handle interacting with the parents from there.
I'd be considering talking to the parents of this other kid, unless you're confident you're likely to get a bad reception. The other alternative - call the cops but ask for mediation, not punishment. See if the police can act as liaison to help resolve any problems, rather than simply having the other boy charged. But if the police try all this to no avail, you may have to accept that having this other kid charged may be the only option.
Hope not, but at least a talk to the police could set your mind at rest. or not - I really don't know how it works for you. But for us - we've been told to call the police now, if another kid even is so much as rude to difficult child 3, without making any physical contact.
And yes, I recall going to talk to a parent whose son had attacked difficult child 3 and bloodied his nose (unprovoked attack) and was stuck with the "he said, she said" situation; "your son started it" when I knew he had not, but had no proof (then). And I was told by this other mother that a few years previously, a teacher (she refused to say who, but implied a female one) had told her of an incident where my son had attacked her son and the teacher had advised her strongly to call the police and press charges. The mother said she knew of difficult child 3's problems and so chose to not call the police at that time (her implication being, 'your son is not always so innocent but I have been sympathetic, so far').
So talking to the other parent, while it can be useful (in our case, we permanently stopped the fighting between our boys) can also be misleading and distressing. I did some digging through my records and worked out who the teacher was and what incidents she was referring to (yay for a Communication Book!) and found that difficult child 3 had hit this other kid only after repeatedly claiming this other kid was sticking pins in him, for days, in class and out of class. The teachers would do nothing about it. I also recalled being told about this independently by a visiting adult who had given a lesson in that class and was horrified to see this boy sticking pins in difficult child 3, in the presence of the teacher who again did nothing.
What parents get told by their kids is always going to be a fragment of the truth, if that. A difficult child who is basically honest (like most Pervasive Developmental Disorder (PDD) kids) will still only tell part of the story. But talking to other witnesses can fill in a great deal more. Whenever you talk to the other parent, you need to be aware that the other parent WILL support their child, especially if they believe honestly that their child has done nothing wrong. A lot will still support their child and lie for them, "because family stick together." It's only a very short step from a parent lying to protect her child, to that parent then developing the lie in his/her own mind to the point of believing it or justifying it themselves.
That's why, if the police can offer it, independent mediation by someone experienced can often help resolve these differences and prevent future recurrences, more effectively than us taking matters into our own hands.
Twice I've spoken to the other parent(s). The first time as I have just outlined - it was resolved, eventually. Partly because of our careful conversation and also partly because I went out of my way to say, "we'll leave it here, today we start afresh with no more hard feelings," and I also found ways to praise this other boy for things he was skilled at, in other ways. He realised I was prepared to be friendly and meant it when I said, "A fresh start," even though I had by then discovered the boy had lied to me and his mother about difficult child 3 doing anything to him to provoke the attack.
The next time I spoke to a parent, we tried the same tack. It hasn't worked. The parents (the father, at least, who husband & I spoke to) is NOW polite and friendly when I pass by on my walks, but the man is a lying, cheating bully who is probably bashing his wife and kids and teaching his son to hit other kids. I've tried to be friendly to the boy, making polite conversation about the way he hits a tennis ball with his cricket bat, all round the neighbourhood. "Wow, you are doing well with your accuracy..." [as he almost hits a passing car] "...have you read about how Australia's greatest cricketer, Don Bradman, used to practice this way to develop his skill? Do you want to be a great cricketer one day?"
The first time I said this, the boy muttered something and walked off. Since then, he avoids me or totally ignores me. But I suspect he was the one repeatedly removing easy child 2/difficult child 2's provisional licence tags from her car at about that time (always parked outside our house). No proof, so we've said nothing (just stocked up on spare licence tags, they're free anyway). However, he's never hit difficult child 3 since, nor said anything threatening to him, so maybe it is still a success, of sorts. The mother smiled and said hello to me a week ago.
Mind you, this is a small country village in Australia, where you smile and say G'day to people you've never met, and stop and talk for half an hour to people you may only know by sight. We're a friendly people and this degree of snub from a kid and his mother has had me concerned for them.
So make your decision, but make the right one, for the right reasons. There should be no harm in getting advice from the police, off the record. If your son has contributed to this, the police will find out. Maybe they should?
I can't say whether talking to the boys' mother will be a great idea, or the worst one you've ever had.
But whatever you decide - I hope you can make this stop. I think it needs to, for both the victim's sake, and the bully's. Whichever way round it is.
Marg