@Rowenaravenclawsdiadem - I'm so sorry this happened to you. It's an awful, hateful, unforgivable thing to do to someone else. I don't care if they were only children. It's impossible to comprehend that a person of any age would not be aware that they were doing something horribly cruel.
Your last sentence resonated with me. My childminder was approached by a parent of a girl in my DC's class (reception age), saying that her daughter was always wanting to play with DC but DC would never include her. I felt so mortified for that little girl, and was deeply upset that my own DC was doing this. Not that DC was ganging up with other people and deliberately excluding her: just saying 'no' every time she wanted the two of them to play.
I had conversations with DC, over the course of many weeks until I actually got through, as to how lousy it feels when someone does this. There were great opportunities on days when I heard 'X didn't want to play with me and I was sad', that I could sympathize with how awful that must have felt and then put across the message: 'this is how Y feels when you refuse to play with HER'. I also had a chat with the class teacher, who herself spoke to my DC and undertook to monitor the situation. (Incidentally, no one tried to 'make' DC do anything - just understand how sad this could make another person). DC is now playing happily with said child. Took a few weeks, but we got through.
Class teacher assures me DC is not a bully, which tends to fit with my own view (because what parent wants to see their beloved child in this way)? She said it was normal for that age. But I'm so glad we took it seriously, and that the other child's parent alerted us to the situation. I couldn't bear for my child to be either a victim or a bully, and excluding other children is widely recognized as bullying behaviour. And there is no way, if I have anything to do with it, that I'm going to let him grow up thinking this kind of behaviour is OK.
Why do some parents think 'My little Johnny can do no wrong and would never do a thing like this?' I can't comprehend it at all. It needs nipping in the bud and shutting down, fast.