It's been mostly an issue at the local park playgrounds here for us, once my kids were going out alone. Almost all of the parents have been great when dealing with issues, but can't always find which parents to talk to after something horrible has happened.
My oldest has been tall for his age for years and that has made him a target - groups attacked him, I guess they think it makes them look tough if they take out the big guy who doesn't fight back. My younger daughter was harassed and threatened & would just stand there crying until someone else stepped in for her and brought her home. So, on recommendation, we've been working with all of our kids on a 3-step method: 1, tell attackers to stop politely and leave if possible and tell us, 2. If prevented from leaving/are attacked again, tell them as loudly as possible, warning that if they don't allow them to leave they have a right to defend themselves and leave if possible, 3 if still prevented/attacked further, they have my full blessing to defend themselves. All my kids old enough to go out go with self-defence alarms and we regularly work through what they can do. My oldest has also been taught to use his phone to record people. The main focus is on getting away, but mostly it's on having an action they can and will take themselves. It's worked well for us, none have gone past 2 since.
Few would expect an adult to take some of the abuse kids suffer at the hands of their peers without a physical reaction and I do not expect my kids to just take violence or harassment because of an ideal of non-violence clearly not being kept by those attacking them or the concept that an adult will always be present and willing to step in. The latter is just not true - many of us, myself included, have stories where adults just stood by and the former is just cruel to put on a child's shoulder on top of such violence. It may not be perfect, nothing in life is, but tools for handling conflict are important to teach and those include dealing with violent people when no one is coming and we're at a disadvantage as too many of us are forced to learn on our own.