It's been five years since I graduated college, and to this day I regret not spending that part of my life focusing on that part of life. Fact of the matter is that I realized the things I genuinely enjoyed hacking (compilers, operating systems) had all been done in the 1980's, and I had no interest in all the web stuff that was being done now. At the same time, anything else I might have wanted to do was closed off because of my college grades. You don't realize at 18-19 how much your college grades matter, especially if you wander outside the tech field.
I'm sure lots of kids are more responsible than I was at that age. They can balance hobby projects along with acing classes. But if you're not one of those people, then I suggest either doing school or doing a startup, but not trying to do both at the same time and letting both suffer. There is plenty of time in life for entrepreneurship. And frankly, outside the little Silicon Valley bubble, your chances at starting a successful business will be greatly enhanced by the signaling devices you can earn by focusing single-mindedly on school.
I do always like to point out the irony of the fact that Peter Thiel graduated from Stanford UG and Stanford Law, clerked for a Circuit judge, then went to go work for an investment bank. He was almost thirty by the time he started his first venture! Life is long, the only thing you're rushing towards is death. :D