Give diaphragmatic breathing exercises, even though they might not work since it's easier with young children, but I'd also suggest acting or singing. I personally know a couple of people who stuttered but spoke incredibly when acting and their general speech would then improve. There are lots of singers and actors that stutter but are very fluent when practicing their art. I know because I have a relative who likes cinema and music so each time he discovers an actor or a singer he likes stutters he tells me immediately. I haven't started acting yet because I'm very shy but I should definitely give it a try. My teacher has a brother that stuttered a lot. At 22 he went to Africa for two years to give medical aid. Since it was around the 70s he didn't call very often so when he came back his family was very surprised to hear that he didn't stutter at all anymore. Fun story. So try acting, maybe at your school drama club. (Or go to Africa.)