MIDad has said it well. If there are no deadlines for dues and these folks will now be "involved,'' how about putting a positive spin on it? If they are now members, ask them to help out with something! Get them on board, get them working on behalf of the school community!
The only way to answer this question is to read your bylaws. If your bylaws, or any rules and regulations they refer to, set a deadline for paying dues, then there's a deadline for paying dues. If they don't, then there isn't.
