OP - I really sympathise, but it is not just girls this happens to. My DS was so miserable at school that I'm home educating him for his last year of primary. It has taken me ages to find out what was going on. He was being excluded in football games, boys were pretending to be friendly with him, then running off with his football, he was regularly left out in groups. The final straw was when his so-called 'best friend' started picking on him and encouraging others to exclude him. For whatever reason, my son was a scapegoat and it was just awful for him.
He has a close friend who he still likes to see and has recently decided that he is ready to see another friend from his old school, who has been asking to see him. But, for the past few months, he has been perfectly happy at home. I think it was just a massive relief for him to be out of that awful situation. From talking to other parents and teachers, I have discovered that Year 6 can be a very tricky year, with all sorts of stresses (SATs, transition to new school coming up, some children entering puberty, etc).
I would agree with everyone who says to encourage other friendships outside school, if possible. In the past, I have tried to help DS's friendship issues by inviting children round to our house and for days out, and trying to get him to form friendships outside school, through clubs etc. I have always been available for DS to talk to when he wants to. Sometimes we would have long bedtime chats and that is when he would unburden himself.
My son does want to go to secondary school, as he sees it as a chance to make a new circle of friends.
However, my son's case is maybe an extreme example and I'm certainly not recommending everyone take their child out of school over the usual friendship issues!
Thanks to whoever recommended that book, 'Bullies, Bigmouths and So-called Friends' - I have just ordered a copy.
By the way, my mum tells me that I used to lament my lack of friends regularly as a child. It broke her heart. I know that I always had several close female friends at school, but we were always falling out with each other (jealousy, rivalry, competitiveness, plain boredom, etc). As an adult, none of this is the case. My mum laughs at how many lovely friendships I have and says, "And this is the girl who said she would never have any friends....!"
It's a phase - but a tough one.