How do I go from being diurnal to nocturnal when I go back to school?

Sleep cycles. It often happens when our primary work changes that we are assigned different shifts (including the "graveyard" shift) or our duties require a change in working hours. The best approach, if you know such a change is coming, is to start moving your sleep times by small amounts (say, 1/2 hour earlier) for several days, and then keep advancing to your close to your assigned shift time.