I've been asked to assist in teaching a HS umipring class. I have a pretty good idea of how and what I'm going to be teaching, but I'd like some tips and ideas for lessons.
Thanks...

1. Get a FED rulebook.
2. READ IT!
2. Read it AGAIN!
3. READ IT A THIRD TIME!
4. Now read it without trying to refer to OBR!!!!!
5. If you can get a FED casebook, good - get one.
6. This is not OBR it is FED - read the FED rulebook again!

Ozzy's right if your class is about umpiring HS games.

If, on the other hand, your class is a class for HS kids on how to umpire then you will probably need to ask them where they would like/plan to umpire and work in whatever rules apply. There are differences that will get them in trouble if they apply the wrong rule.

Yes, we have already discussed that and are going to be officiating games covered by Babe Ruth and LL rules.

You can try...
 

teaching some professionalism. (It will be a challenge.)

They will make calls, close calls, that will get the players and parents excited. There will be controversy. They will be completely unable to sell their calls if they are wearing shorts, shinguards, squatting behind home plate, with their hat on backwards, and talking to their girlfriend between pitches.

Of course there is much more to professionalism than just dress and attitude, but get them going in the right direction.