You can always go back to school. Taking some time out on a startup where you're trusted and valued is rarely a bad move.
You should play the school card as a negotiating point, though. I'm usually very upfront in negotiations and would simply say "Look, I've got a lot of pressure to finish school. I totally believe in this company and am willing to put school off in order to join full-time, but you need to come up with a compensation package that makes that worth my while. My priority is on long-term compensation so I'm fine with a super-low hourly wage of $15 per hour in exchange for equity. What can you offer me?"