in the classes or maybe call it Class2Student table. Create a record for every student that takes or is trying to enroll in a class. Then in that table add dates fields to track the enrollment process example add LetterSentDate field. If you sent a letter then it have a date. If you haven't yet then it would be null. use a boolean field for Not excepting example isNotAccepting = false, and last one AcceptDate is null but check so that you don;t have a date and isNotAccepting is true. I hope this helps.
Al
Al