First of all, kids are dumb. I know it hurts right now. But, kids are dumb. If they wanted to mock you, they would've found something. If it wasn't for your stutter, it would've been your hair, weight, clothes etc. Anything they can pick. Sometimes, people will just say shit to try and rile you up, and to try and make themselves look/feel good. Personally, when people mock my stutter, I usually just say, 'Yeh. I've got a stutter. That's how I talk.' It's like that Tyrion Lannister dude said, 'Never forget who you are. The rest of the world will not. Wear it like armour, and it can never be used to hurt you'.