*Bubble, the OP asked if she WBUV unreasonable but in the initial post gave literally no information so it was absolutely impossible to say whether or not she WBU. She has gradually given more info but we've needed to ask a lot of questions to get it. And yes, stuff like was it part of a campaign of bullying or if the other girl was injured are very relevant to the OPs question. How she deals with this very much depends on the level of culpability and how serious it was
As someone said above, fights are part and parcel of school days so instant expulsion seems like it must have been a pretty serious incident (although the OP has avoided answering what sort of injuries the other girl was left with. And it does seem now like the OP attacked the other girl.
I think the OP is potentially being given really shit advice. If the other girl was quite badly injured and remains resentful that the OP wasn't prosecuted (as she says there was a feeling she 'got off lightly') the advice she's being given is probably the absolute worst she could get.
If there are people who are still shocked by her actions and if they had a lasting impact for the other girl involved or she was badly injured, then the OP breezily insisting it was years ago, it doesn't matter anymore, is all trivial and they're just nasty gossips is going to make them think she's a callous cunt who hasn't changed, doesn't have any insight into what happened and is to be avoided.
What the OP has actually said on here that she feels about the incident anyway is a far, far better way of truthfully dealing with it. It she fronts this mother up and says it's a huge regret of hers, she's very ashamed of it, it happened under difficult circumstances and doesn't reflect who she is now - the other mother is going to look much more favourably on her as acknowledging what happened, knowing it was wrong and regretting it.
I think it's a terrible habit of posters on here who are in such a headlong rush to tell a poster that they're in the right because they think it's nice or kind. But they don't consider that they might actually be telling the poster to do something incredibly destructive if they're not actually 100% in the right. And I think this may well be one of those occasions when the OP would cause even bigger problems for herself if she takes the advice of giving the appearance she did nothing wrong and doesn't care.