If the student is 18 and has graduated can a teacher date him? Is there a time period after graduation that they should wait?
Lawsuit / Dispute Attorney
Does the school have a non alumni dating policy?
Divorce / Separation Lawyer
If you are asking about any sort of trouble, then yes, it is possible the high school teacher can "get in trouble" for dating a recent graduate. The time they should wait cannot be determined, but given the fact that it appears this relationship was going on or imminent before graduation, this is a very foolish thing to consider.