If you count Woz as "still at UCB" because he was on a leave of absence, then you have to count Gates as "still at Harvard" and Page & Brin as "still at Stanford" even today. Officially, they never actually dropped out of their universities - they just remain on perpetual leave of absence, and could go back next semester if they wanted to. Woz was at HP because he didn't have money for the next semester of Berkeley, and he thought he would always be at HP, and even after Apple started selling things and went public, he remained officially on a leave of absence, and actually did go back after his plane crash.
It's actually really hard to drop out or fail out of an elite college - I tried a couple times when I was still in undergrad. They always want you to take a leave of absence because then you don't show up in the dropout statistics that drag down their U.S. News rankings. This is why it's not really all that risky to drop out of college; you can always go back and pick up where you left off, but you'll have a much better idea of why you're there.
I was definitely thinking about Evan Williams for Twitter. He dropped out of college because he always knew he wanted to be an entrepreneur, but had no idea what sort of company he would start. I don't really know about Dorsey and Biz Stone's backgrounds.