paying instructors when they're on the board

Suppose you have a nonprofit organization whose express purpose is to teach PDCs. Suppose the board of this nonprofit consists of people who care about and want to teach PDCs. Now suppose the organization hosts a PDC and wants to pay the instructors for their work -- not from the general funds, but specifically from the proceeds of the class -- and the instructors happen to be board members. Is that a conflict of interest? How can legal unpleasantness be avoided in this situation?

I don't think so, but it just takes one person to make something out of nothing. And how hard would they have to dig to even find out?