Off the top of my head, I'd guess we've had about 8 startups where the founders went back to school. It doesn't only happen with summer batches. Founders from winter batches do it too.
Usually the reason is that the startup isn't doing very well. However, that judgement depends a lot on how determined the founders are. One reason we now shy away from funding people still in school is that they often unconsciously want the startup to fail, because the idea of dropping out frightens them.
A lot of startups look bad after 3 months. Someone who's out of school and has to make it work or get a job in a cubicle will say "don't worry, we'll figure out how to make it successful."