Welcome to Fluther.
For starters, you’re “the new kid” in school, so this kind of thing is going to happen from time to time. As others have said, part of it is a natural animal desire to establish a pecking order, to see where you fit in. If you cave, cry, get upset or otherwise show them that “they’ve won” when they pick on you in this way, then they know “we’re still on top”. Again, as others may have mentioned, but which can’t be stressed too much “it’s not about you”. That is, don’t take it so personally that they have it in “for you”. As you already know, they don’t even know you!
Other than that, if you look particularly good (or bad, unfortunately; we need to face facts) to them, then that’s going to fuel their desire to assert dominance.
If you can lessen your own emotional response to their comments, then even if it doesn’t cause them to stop making them, at least it will take some of the sting out.
But really, welcome to Fluther. We’re not like that at all. Sometimes we’re worse, actually. But I’ll be nice. Promise.