At this question, there was a comment:
Labor laws are likely to forbid having anyone teach for free, as are union contracts where applicable.
The situation is that someone is contemplating going to grad school with support, meaning a tuition deferment and a stipend, in the form of a fellowship or research assistantship instead of a teaching assistantship. He would like to know whether it would be legal for him to do some teaching during an occasional semester, as though his support were in the form of a teaching assistantship.
Are there labor laws in the U.S. that would prevent him from doing this?