a flu is a crap reason tbh unless it is SIGNIFICANTLY affecting your ability to sit in a room and put pen to paper for a few hours.
the flu isn't a crap reason, it can actually get pretty bad. do not confuse the flu with a cold - completely different things that everyone thinks are the same.
i had it earlier this year, went on tamiflu - muscles ached, was very tired, severe headahces, sore throat, cough etc - you just don't want to move or do anything
i'm glad i've had it already and not now.. that's really unlucky.
also, if you have it, you are still infectious until 3 days after you started your course of tamiflu (the safe time) so i am unsure about how this would affect you being able to do it at school.. i would agree with imsopostmodern, and call your school