You may have been somewhat depressed before- and depression will definitely decrease sexual desire. You may also have become physically run down- with depleted energy and not eating enough, you could beceom physically exhausted and under nourished. If you don't have enough fuel, protein etc- you run the risk of actually starving yourself- which will depress both your general energy and libido.
If you have gotten yourself into some kind of destuctive relationship to your body, you need to break that cycle. I would reccommend you see a doctor to see if your body weight is below what is normal for your height and build. If it is, you should see a nutritionist to help you build up muscle and energy.
If you are not underbody weight, you could still be depressed- and might need a mood mediator like prozac of zoloft, eflexor , etc to feel better. There are behavioral answers to: perhaps you are so full of anxiety about school that you can't relate to sexual expression. You need to see a counselor to find a way to relax, enjoy life again and ease up on yourself. I think if you get your mind and body together- your sex drvie will come back.