I personally would charge by the head. If the community is providing a place to teach classes at no charge, I would charge the going rate at the nearest facilities per person. If you do a good job, you could do quite well. Especially if you can get multiple classes going.
If the community wanted to pay me an hourly fee, it would be my middle rate for small classes and my higher rate for large classes. $50 to $100 for me personally.
Now if it was a lower income area, I would probably only charge $1 per person for legal and insurance reasons. I do some of this type of work as well.