When we started Viaweb, I was done with grad school but Rtm and Trevor were still in it. That worked out ok. Grad student procrastination is actually quite a powerful force. It's ok if some founders are still in school, so long as (a) the ringleader isn't and (b) the ones still in school are willing to put a good deal of time into the startup.

You could wait a couple years, when it's a good bet that 90% of your college friends will want to leave grad school. They'll take much less convincing then.

In a couple of years that business idea will surely be done by someone else. I think that the best time for doing something is now.