Oh dear! We're having exactly the same problem here with DD. She's in Y6 too.
She has been upset for a few weeks now, and also the teacher had a word yesterday about problems with her behaviour in class, which has never been an issue before. She seems to be messing about to try to impress some of the other girls.
She said tonight she feels like they are all 5 years older than her. I thought that was quite telling, because they clearly are all growing up fast, and she feels really out of her depth.
I don't know what the answer is. I've just had a big conversation with my DD at bedtime, and we've agreed the following:
1: For her to stop and think if she's being oversensitive - ie are they actually being mean, or just laughing at something that she could join in with. Do they laugh at each other in the same way, or is she really being singled out and targeted more than others.
2: To make more effort herself to work on friendships with her old friends, and in particular her best friend, who she's drifted away from.
3: To avoid the kids who are causing a problem, but not to actually make enemies of them.
4: If she feels she's being targeted / singled out / bullied, to make sure she tells me, or the teacher.
5: Not to join in or start any nasty behaviour because apart from making other people feel bad, it means you become more of a victim yourself.
I have no idea if any of this will work. It's all easy to say, and she did see the logic of it, but whether she can actually do it is another matter.
Good luck!