If you still have shame and embarrassment about your stutter, it's hard and there's no quick easy answer. But even if you have shame and their mocking feels bad, at least you aren't that lame of a person to make fun of someone for something they can't control. You may stutter and you may feel embarrassed by it, but it's something you do, it's not all that you are. The kids who make those kinds of jokes, that comes from part of who they are and it's totally weak. just remember you're a better person than them and that temporary bad feeling you may feel isn't as bad as having that kind of personality. I'd feel sorry for them, you sound great.