1) Large amounts of free time. You've got a six to eight hour or so block of time every night, every weekend. You can probably pull programming books into class with you if you're that insatiable. If not, just carry around a notebook with you dedicated to the startup, it'll be chalk full of ideas whenever you have a spare moment.
2) No responsibility. Granted, high school is tough, but it's also inconsequential. If you've studied the PG essays as much as you think, you'll remember the concept of a day job in "What You'll Wish You'd Known". Here's the link, it's worth a reread in your situation. http://www.paulgraham.com/hs.html
3) Your parents are funding you. They pay for your housing, food, and are probably quite open to the idea of buying you and books you want to pursue your goals. You can afford to take the time to learn right now. If you drop out, are your folks going to want to keep feeding you while you sit around the house? You'd probably be expected to get a job, which cuts into all that free time you'd be gaining by dropping out.
Delay University if you want, but honestly you can get a lot of the same deal out of it if you approach it well. Right now you've got an idea and no skills with which to execute it. Don't create a situation that's more hostile to you getting those skills.