Well done on being chosen to do this. It's great that you're getting this kind of experience - and you sound very determined. How big is the class? One thing I find helpful is to tell people early on that you have a stutter (i.e. on day one as you introduce yourself). It doesn't have to be much, e.g. "I have a stutter. It's not a problem for me but sometimes it'll take me longer to get a point across. If you don't understand me just let me know". By taking ownership of it early on, it defuses a lot of the tension around worrying whether you will stutter. It also puts people at ease to know that you can be open about it.