Because you don't know what will happen, and depression is a terrible disease. I'm a recovered depressive myself. When you go back to school, be sure and have plenty of medication, get your exercise, get plenty of omega 3 fatty acids, and make sure you have emotional support. If possible go to a school in your home town, so you can be with your family.
School has always been a stressful environment for me. I have some social anxiety which makes me feel a little depressed sometimes. However once you're in school for a few weeks, you'll fall into the swing of things and everything won't seem so bad. So hang in there :) Everything will be alright.