Girls do go thru' mean phases - my dd learned how to cope with this in Yr. 6 when she was on the receiving end.
However, when there was some nastiness between dd and one of her friends, friend's mum decided to go into school, and complain about dd making her daughter upset.
The school clearly felt that it was a storm in a teacup, and didn't bother to mention it to me, until dd spoke to me, and I contacted them.
I said that I felt the girls needed to sort it out, and unless it moved on to bullying, I would keep out of it. School endorsed this approach.
The problem was that friends' mum really thought (thinks?!!) her dd is perfect, and could simply never be nasty. Not true - they all can.
But, given time, they work it out between them, and I really think that mums getting involved can make the situation more more fragile.