So I recently just graduated, and I worked (at internships during school AND summers, pretty much ~30 hrs a week), and I had plenty of free time, I just used it unwisely (generally).
I haven't worked in a startup, so I can't really give you the most accurate of time measurements, but if you're working on something you like, I'm pretty sure you won't even feel 50 hrs a week go by. Also, in a company that is fully aware that you are still in school shouldn't really be expecting 60 hours. And if they do, you can use that as leverage for more flexible working locations (like the ability to do most of your work from home/on-the-go). But generally it doesn't seem like any reasonable person would expect you to work 60 hours (as a new hire) while going to school. Doesn't seem healthy