I wouldn't say "impossible" because I'm pretty sure someone out there has done so. But it's highly unlikely for most people to do well (or even pass) if they try to take a full course load -and- work 50 hours per week.
Personally, I had a little trouble keeping up working full time and taking just one night class at the community college (although they were five-credit classes in engineering calculus, so nearly the equivalent of two three-credit classes). Not that I didn't end up with As, but I had to take a half-day vacations the afternoon before exams to study more often then not! I had a pretty flexible (but fast-paced job) as a government attorney.
You need to choose your priority. If it's work, then take classes no more than part-time (two classes a semester sounds reasonable; or approximately six credits per semester).
It's not a permanent decision. If, after a couple of semesters, you are getting the grades you want, not missing any work, and still have time to spare, then maybe add a class. If you still are getting the grades you want, not missing any work, with time to spare, then consider going full-time (remember to amend your FAFSA at this point, so you can get more financial aid).