I second cronker's suggestion. I work in a very stressful field right now, and if I worked out only when I didn't feel stressed, I'd never make it to the gym. If you are in school, your body will need to perform in top condition, especially if you're working on top of it. Although you are often tired after a trip to the gym, working out will actually give you energy, which, combined with proper sleep and planned rest periods, will help you to be more energetic during the day, as well as sleep better at night.
As a recent graduate, I fully understand your predicament with respect to costs of gym membership and personal trainer. Check the class catalog at your school to see what kind of phys ed classes they offer. You may find yoga, pilates, or some other activity that will help you to strengthen and tone, while also encouraging your emotional/mental/spiritual well-being. You may also try a nutrition class. Having a background in nutrition will put you in tune with what kinds of fuel your body needs at a given time. We are an exact product of what we put into ourselves, on many levels. There are even great articles on this website that will show you what to eat to help combat stress. [Here's a tip: McDonalds is not on that list.]
Hope some of this helps.