Well, I think the best training material is this forum, but Brad's not paying me or anything.
I'm starting teaching high school, too (although I will not, unfortunately, be teaching a reffing class), and I took a football officiating class in college.
My first recommendation is that you require everyone to get a copy of the relevant rulebook(s). There's no substitute to actually reading and learning the rules - particularly if this class is for academic credit.
As for training materials, I'd do a Google search for college intramural sports programs. At the least, most have 'dumbed-down' intramural rules, which may be a good place to start if your students have no sports/officiating experience whatsoever. (I know GWU's website -
http://www.gwu.edu/~imsports - has these.) With some looking, I bet you could find some programs with guidelines/training suggestions listed.
One in-class or out-of-class activity that can be a good tool (especially for kids who are good athletes but new to reffing) is to have the students watch a game and focus on the reffing. In class, you could tape a game over the weekend, possibly edit it into key plays, then watch it in class and discuss different calls. You can use replay and slow-motion as much as you want, and let the kids use the rulebooks to find the correct answers. Alternatively, the students could be assigned to watch a game on their own (in person or on TV) and write a mini-report focusing on the officiating during the game.
Another thing I'd suggest is talking to officials you know, and seeing if the students can shadow them during games - probably no more than one or two students a game, though. Also, you could talk to the AD and see if he/she needs assistance with "hosting" refs at your school's athletic events - perhaps you could assign this on a rotating basis, or make it for extra credit.
Let me know if you need any (more) help, and I'd love to get ideas/see how this goes. If you want, you can e-mail me through the forum.