There seems to be a lot of great comic resources floating around the internet recently, and here's another good one. Mark Kennedy's Blog is a fantastic resource for drawing tips and skills. If you look at any of his posts that are begin "D&D" (that's 'Drawing and Design', not 'Dungeons and Dragons'), you'll find some excellent tips on compositional skills that are useful for both cartooning lessons as well as any kind of drawing lesson.

Also useful is the Comic Strip Artists Kit, which Disney artist Carson Van Osten drew in 1975.

As far as using these resources in a classroom, I think that its important to note that Kennedy and Osten both rely heavily on ideas of what "works" and what "doesn't work". While they provide very useful examples and interesting ideas, they don't leave a lot of room for thinking outside the box. So, as a classroom teacher, I'd feel more comfortable introducing these excellent resources with an explanation:

"These are great resources and provide rules that make a lot of sense. Read it. Understand it. Try it out. Figure out why their rules work. Then try and break them."

Just a thought.