I am all for volunteering and the such. I also think it is very important to find out if the organization who is asking you to “volunteer” your time is a non-profit organization. If the students are paying and you’re “volunteering” your teaching, then I personally would be a bit hesitant to “volunteer” in the first place.
Let’s say it’s a summer camp, I would be sure to ask, “Are the students paying to come to this camp?”. 