My opinion on this is that it depends what type of pharmacy you want to work in later on: hospital or retail. For residency programs, work experience is a plus, but you definitely need a solid GPA and organizational involvement as well. So if this was your case, I would say skipping out on the job to focus on school would be your best decision. But from your post, it sounds like you're more interested in retail/community pharmacy. Work experience is DEFINITELY what you want for this field. We had a career fair at my pharmacy school, and I heard many students got job offers because they already worked with a company. Especially Walgreens and Kroger (although it was also the case for many other chains as well). Although I don't have any actual experience job-hunting as a pharmacist, I would think that although it wouldn't be impossible to find a job without having worked during pharmacy school, it will likely make it more challenging for you. It would probably involve a lot more phone calls and pavement pounding and working your way up with temp jobs/floater positions (which might end up being the case even if you do work during our current job market🤔🤔🤔).
If you're having trouble passing classes, I would still skip out on the job, because all the work experience in the world doesn't mean anything if you don't have a degree. But if it's just your GPA you're worried about, most community pharmacies don't place a high emphasis on grades.
I can also say that how much you're expected to work while a student varies SIGNIFICANTLY among pharmacies. Some pharmacies want you to work 20 hrs+/week, and some are okay with 2-4 times per month. If you know any insiders personally, definitely ask how much they're expected to work. Or if you don't know anyone, just be open during the interview that you can't do more than 10-12 hrs/week (or whatever you think you can do).
Hope this helps!