I'm taking 17 credit hours, and working two jobs. One at my school, and one near home on the weekends. Its possible, but you arent going to get much sleep and have little social life, or you're gonna get 4 hours of sleep a night and have a social life.
So, its possible, but not easy, to go full time and work 40 hours a week. I think once I finally get my @ss out of debt (which should be made official after my next paycheck), I'm going to only work 1 day a week at the second job, or maybe drop it altogether, because I'm getting little sleep, and and have almost no social life right now.
Going to school full time and working part time is what most people that I know end up doing.