I'd imagine that your school equipped you for this, but the most basic thing I think is to assure kids is that it's OK to mess up. No pressure or anything like that. That tends to make to worse. Encourage them to express themselves even if they stutter. I haven't been in much therapy, but it mostly focused on "fixing" me by reading out loud a lot. Needless to say it didn't work. Besides that, I'm not sure what would help. It's generally tough being a kid with a bad stutter and I don't think people can really prepare kids for what it's like to be bullied over it.