I totally know how you feel! I felt the same way (and still do hehe) in high school.
It might help if you can find a way to get some exercise into your normal routine. Do you live close enough to walk or bike to school? I lived about a half hour walk from school, so that was an hour a day of fast walking that I automatically got!
What about joining a team or sport? I did this in HS and we had practice 3 days a week and 2 games per week, so that was an additional hour and a half of running per day.
Also, are you getting enough sleep? If not, that could be why you're so tired.
Edit: I have a bone to pick about the "real world" comments! Adults don't understand student life these days (of a tgood student anyways). You can't just get good grades and expect to get into a good uni anymore! You need to have near-perfect grades, be on varsity sports, volunteer, organize fundraisers, travel, play an instrument and have a job and still somehow have time for friends and family which then includes contributing to the household by cleaning, cooking, watching younger siblings etc.
If you add up all your engagements for the week, a student can easily have 100+ hours of "work" per week, and then you have to fit in friends, family and exercise. And sleep (which is always the first thing to go)!!!!
Add to that the stress of a senior who has to make college decisions, do applications etc.
This is coming from someone with a "real world" summer job for the past 4 months. Let me tell you - the "real world" is infinitely eaiser (albeit more boring and depressing) than being a student.
OK, that's my rant