Do you mean a few days off a semester, or a few days off a week?
Unfortunately, your schedule (for the most part) is all set for you. Unless there are electives, you don't really get to pick what you take. Time in class/lab does vary by school, but most have a pretty full schedule (9am-3/4pm give or take a few hours here and there), so if you are planning to work a substantial amount, it might be difficult to balance school and working. Taking a few days off a semester (so skipping class/lab) could be possible I suppose, though missing a few days a week to work could be problematic/discouraged.
On a personal note, I don't think it would be possible for me to get a job at the moment due to the sheer amount of studying required. However, there are several in my class who work 5-10 hours a week outside of class and find it doable, so it's all up to you and how you adjust to the curriculum.