Learn English – What would you call someone who sneaks into and stays in places where they have to be signed up, e.g. classes/country clubs

If you purchase an item from a store and they do not want to give you a refund but they allow exchanges, there are a couple of words for this including an "exchange" or giving you "store credit".

If you reserve a tennis court and they won't give you a refund but they'll allow you to change your reservation to a later date, I think this is simply "rescheduling" or "changing your reservation". It would appear they also have a policy of "no refunds".

As a general rule, they aren't called teachers at or above the university level, they are called instructors or professors. The term you are looking for is adjunct, and you could call this person an adjunct professor, or an adjunct faculty member.

NOAD defines the word like this:

adjunct (adj.) (of an academic post) attached to the staff of a college in a temporary or assistant capacity : an adjunct professor of entomology. [as n.] both adjuncts and tenured professors tend to inflate grades.