For stress, I always go out for a run. I've never been the athletic type, I always hated sports, but when I feel so stressed out or under pressure, I go out for a long jog. To me, it feels free to just escape from everything, take a long time alone and have the wind blow through your hair. (Deep, right?) You can take a tour around the whole neighborhood or a portion of the city, but I like to stop at parks and swing on the swings.
I agree with Yume about playing games. I was extremely obsessed with video games it was so ridiculous. Stay away from games if you are busy, use that time to study more for a test or take a rest from all your work if you feel like you need it.
If you feel like video games help you relax, then keep going. For some people, time online can be a great place to take some time out from the real world and just enjoy yourself.
I hope you are not, but if you are obsessed with video games like I was, then force yourself to stop. Cut off the internet, give your computer away to a close friend for safekeeping, do something. If you have free time, you can always draw or read or write. Going back to games might get you hooked again.
Be optimistic. Just look on the bright side; if you think that something is going to be horrible, it's probably going to end up being horrible because you thought it was going to be. If you study, prepare, work out, whatever, for a big event, you're going to be fine if you prioritize your time right and try your hardest.
On the topic of keeping a schedule, manage your time so everything will work out. If you do this, you'll see progress, and you wouldn't have to worry about it anymore.
DO NOT PUSH YOURSELF. Some people argue this with me, but I absolutely agree that you should never push yourself.
Unless you are doing bad or see a fall in your grades, do not work more than you should. This will just add stress and more pressure at being perfect. You do not have to be perfect and you do not have to push yourself. Work hard, but not too much. You do not have to go over your limit in order to be amazing.