The thought of leaving school deeply saddens me...?

Nope. It is completely normal. You are realizing that your life is going to change COMPLETELY after high school. And good for you, because a lot of your peers wont understand that until things get really tough. I'm graduating a year earlier than everybody I know, and the other day I almost cried because I realized all of the things I'll be missing out on: Prom, my senior year, a real graduation. But it is all worth it. Do you know why? Because you are maturing. You are getting your priorities in check. Like I said.. good for you :).