I'm sorry this happened ro you, I had a similar experience when I was close to your age. An adult asked me what my name was, and I replied K..K.. Kelly. Then it was "hi K..K..Kelly! Hahahah. " I told my mum and she said the woman would never have done that if she had been been there, which is true because bullies will always target people who they think they can get away with doing it to, as you get older it becomes less socially acceptable to bully people for things like stuttering, but bullies always find a way to bully. Working on how you come across such as standing confidently and remaining externally calm even when you stutter has helped me, some people just look like easy targets and once you've been bullied, you can subconsciously start to think of yourself as a victim and come across like that, try to do something to boost you're confidence and self esteem if you can. In the moment, make sure you're not giving the desired reaction, ignoring it is good and if you can find a way to make to make them feel dumb for bullying you, even a facial expression that says "what a weiredo" can help diffuse the situation. At the end of the day, mocking a stutter is idiot level humour, and it's not funny to anyone with any intelligence. Most importantly, no matter how much it hurts, don't show them it bothers you, then you give them what they want, to feel superior.