Im in high school too and stammer, its nerve-wrecking. Tbh, practice alone and if when you are up there just say it like you are all alone. stutters will come here and there but it will certainly be less than if you intimidate yourself. dont feel bad also if you end up awfully stammering. stammering is a time game and will gradually heal more and more over time. eventually u will be able to talk with 90% fleuncy on everyday things, guarantee it mate!