I'd generally say no (I think some school's even have rules against this.) Between class and studying, school will already be more hours than most full-time jobs. Putting another 10-12 hours of work on top of this doesn't seem wise. It's 10-12 hours that could be spent studying, engaging in an activity that could help strengthen your profile, or even just resting to help prevent burnout. Not to mention, the amount of money you'd make in most part time jobs is going to be so small in comparison to the size of your loan that it's basically immaterial.
I guess there are occasional exceptions. I know someone who's an RN who picks up shifts. I think the flexibility of that plus the fact that the hourly pay is sufficiently high makes that make sense for her.