You can calm yourself down when you are extremely stressed by taking deep breaths which will help your relieve some of the physical tension. You can also take a walk so as to clear your head and try to sort out the things that are bothering you. You can get more tips on http://stress.about.com/od/tensiontamers/f/calm_down.htm.
Do your homework as soon as you get home, have smart friends, a significant other, and if you're pushing to hard, say **** it and let it slide (only if it won't effect you too much). Stress causes Cancer man..