If the absense is planned ahead of time, notify your course coordinator of your absense, so that someone can be lined up to substitute for you. Regular absenses are not acceptable.
Emergencies happen, so it is possible you might miss class unexpectedly. If this happens, you should call the Mathematics Department Chair as soon as possible to notify him of your problem.
Only the Chair or Assistant Chair of the Department have the authority to allow students into closed sections. Explain this to the student and send him or her to see the Assistant Chair.