As a former youth educator, I have seen kids be mean and it's disheartening when it is intentional...as it seems with your son.
I would first, talk to the teacher. If the behavior has been repetitive and nothing has been done then these, 'mean kids' are getting away with 'bad behavior'. When I was working with kids, it helped me to hear from the parents when they were concerned. Honestly, at this age I love seeing kids who can stand up for themselves, and use their words to express when they are being mistreated. It's the kids that are afraid to speak up, that I feel for, as they are picked on and intentionally singled out by others for most of their lives.
My son is 2 years old, and I tell him even today, if someone does anything that makes you uncomfortable or scared or mad you can tell them 'stop it, I don't like that'. I know he's young, but I want to start now telling him that he can come to me about ANYTHING!!! It's funny to watch when Grandma is tickling him, and he tells her 'stop Grandma, I don't like it' when he isn't in the mood. But, it's also empowering as a parent to know that he is going to be able to stick up for himself.
Make sure you have an open dialogue with his teacher, she may not know that this is happening. Ask the teacher, how they deal with discipline issues and what is the best course of action to take if it becomes a persistent issue with the same kids and your son. Usually, schools have escalation paths they take for this kind of stuff, and can help mediate these situations before parents who don't know each duke it out over their kids. If you know the mother, maybe casually ask if you have a chat about your kiddos.
It's worth taking action now, so your son doesn't get put off to school.
Good Luck.