I like the clean the barn party idea......I think that is great! WE do that for trail maintenance too! Personally, with respect to this incident-I would not clean during their lesson-that is really passive aggressive, IMO. I would say something to the instructor for sure, not the clients. To them, I might just suggest that they ride with brooms or towels. That way they can do their designs, clean and "bombproof" all at once. Multitasking.