At Viaweb, two of the three main hackers (Robert and Trevor) were in grad school the whole time and actually got their degrees. In fact, Trevor got his a day or so before the Yahoo deal closed, and was the subject of Viaweb's last press release:

http://www.paulgraham.com/tlbphd.html

I believe some cofounders can still be in grad school, as long as at least one isn't.