I went to a mixed school and still had negative experiences with girls, but my sister (also in my mixed school just 3 years above) had an easy time in school. After leaving school people have said that I was in a particularly nasty year group so perhaps if you moved her into another school and she got lucky with who else was in her year it would make a difference, but it is luck of the draw.
I was the same, very loyal and got very upset when they went against me and looking back definitely believe my reactions made them gang up on me more. Occasionally the group of girls I was in would try and turn against one of the stronger girls in the group and it would never be as bitter or as dragged out as when they did it to me, because I'd cry and argue back and be the one walking around school on my own visibly upset, going home pretending to be ill, staying off school lying to my mum I was sick etc.. those who just stood up for themselves were a lot more respected and it is so hard to do that at a young age. As I grew up and out of my social anxiety and wasn't scared to stand up for myself those problems ceased to exist, obviously they lessen leaving school but I do know girls my age who still have issues.
I find in school it's always large group of girls as well, and there lies a problem in itself as not everyone will like everyone, there will be certain people who don't like certain people, and when those certain people are stronger characters who take a dislike to perhaps more anxious, socially awkward members of the group it's easier for them to be excluded when that person is the one making the plans, running the show etc. It's better to do have two of three close friends, then ten fake ones and it took me a long time to learn that but I'm far happier for it.
I feel like I'm not really giving much advice here but I just know from my experience nothing my mum could of done would of helped. It was me who had to make the changes and realise what I deserved in order for problems to stop. It didn't mean those problems disappeared I'd just stop caring about them and the people causing them.
Facebook, Instagram etc weren't really used by my age group in school - it was the MSN and bebo days and although you still had the bitchiness it was to a lesser degree, I wasn't allowed to use the computer for more than an hour a night, most of my friends were the same and no one really had phones that could use websites either. There's so many avenues for it these days and it's not limited as much due to phones so it's far harder to escape 