Chatting it up with your instructor prior to class is an easy way to get more out of your workout. Keep them in the loop on these occasions, and you will get a more efficient workout in return.
Dealing with injury: Whether you're just coming off of an injury or have a preexisting one, it's important to let your instructor be aware of it. If you wait until the middle of class, you risk overcorrections by an instructor or a position in class that you simply cannot get into. Just briefly remind them before class so that they can modify the exercises to be much more beneficial for your needs.
A late start: If you can tell that your class is going to be late because the one beforehand is running over or the instructor is tardy, don't be afraid to let him or her know that you need to leave on time. This will make both of you feel more comfortable because you won't need to tip-toe out. Teachers get that you have commitments, so they'll understand if you need to leave at the appropriate said time. Along the same lines, also give them the courtesy of alerting them if you have to leave class before its scheduled end.
Welcome back: Coming from the flu or anything that might make you feel dizzy in class? This is a definite update to let your instructor know about. Anytime you return to exercise after an illness or even jet lag, you can be surprised by your stamina. Tell your teacher before class so that you can take breaks when need be or slow down your pace during the exercise in class. This way you won't feel guilty about "wimping out" and you can avoid pushing yourself beyond your limitations.
Move confusion: If you think you've been doing a move wrong because it feels funny or you don't feel the burn, bring it up before class. By mentioning it to the teacher prior to the start of class, they can correct you during the movement while it's still fresh in their brain. They can also take mental notes and let you know after class what they think the issue may be. This is much more efficient than telling the instructor afterward because they'll get a chance to be hands on with you the same day the issue is brought up.