Thank you all for answering and giving support. I just want to cry and throw up, I feel so sick! Such good advice too!
When my DD told us (her dad was here too), we stayed calm and asked non leading questions (where, when, who, what, how) and kept it calm, but showed her we took her seriously and believed her. I said that in this instance, it is OK for her to push him away and also to shout really loudly at him "No" or "Go away", but she said she felt embarrassed and didn't want others to hear (I feel sooooo sick writing this down!). I told her that I was glad she told us and that I was proud of her for remembering what I taught her (telling a safe adult if someone does anything of the sort). I reminded her she's safe with us, and that I will make sure her teachers also keep her safe at school. I didn't show big emotions or anything like that, I didn't want her to think that telling me such things would create big reactions and risking her not wanting to share in the future, but inside, I'm a fucking mess! Thank you all for making me feel like I'm not overreacting.
I then called the school straight away (it was almost 4pm so minutes before they stop answering the phone) and the receptionist passed me straight away to the teacher (I don't know if they're the DSL, I was quite shaky and I was trying to sound firm and authoritative, so didn't ask many question, just stated what happened and how unacceptable). Teacher sounded like they took me seriously and wanted to deal with this asap (said they'd contact the parents straight away, and talk also to the child, then focus tomorrow on boundaries and private parts, also saying they would be extra vigilant). Teacher also said they'd talk to me tomorrow.
I'll take on your advice and ask to talk 1st thing + ask specifics of how they will keep my daughter safe (can they separate him from her in class, in the playground and at lunch time, potentially changing him class) + what happens if the behaviour doesn't stop?
My DD is quite a strong, confident person and she doesn't seem scared or anything (she seems more annoyed with him, like he's a pest) but she should NOT have to deal with this, ever! Also, again, what if this behaviour doesn't stop, gets worse, then what?!?!!? Seriously already thinking of changing her school, but she would be devastated, all her friends go there (we live on the same street as the school and are friends with neighbours, kids from nursery, etc. Didn't know this boy before school though).
I hope this was just a case of the boy being curious and crossing boundaries, and a firm talking to by parents and teachers + education on the matter will be enough, and not something more sinister.
Sorry for the long posts, I find it easier to write it all down here than talk about it irl, I'm still so shaken by it all!