My experience was quite similar to sasha's.
I was bullied from the age of about 7 years old. And it got progessively worse until the day I left Secondary School. My bullying was both mental, verbal and physical.
The main couple of reasons I was bullied was because I was different- I came from a two parent family, with their own business, a big house and a holiday home. I got given everything I wanted, got a long with adults a lot better and was extremely well-spoken.
Is your daughter an only child? As it happens a lot more regularly to only children compared to children with siblings.
This is just my thoughts anyway.
My advice would be to support your daughter, tell her to walk away from them, as she is better off alone than without some 'friends' like them.
Definitely speak to the teachers, and whatever you do, if it continues, don't give up. Even if you have to go back everyday. Because they probably ignore it as just 'normal' reactions.
I would keep your relationship with your daughter as close as possible, so if anything else happens she will tell you.
As I didn't, and ended up in a very bad state.
Tell her not to be beaten down by these sh*ts, otherwise she'll continue into secondary school with a 'beaten' attitude and will continue to be bullied.
Maybe, when she's a little older, pass her onto a counsellor, so she can speak about these problems if they continue.