I've known of a few employers that paid up front (by quarter or semester) as their employees were going through school, but they required the employees to sign a contract regarding how long they would be obligated to continue their employment or have to pay the money back. In my experience, most employers (if they offer any kind of tuition support at all, which they're not obligated to do) reimburse after the employee has successfully completed each semester.
This was popular back in the 80's at a time when there was a shortage of registered nurses, at least in my area, anyway. The nursing classes were pre-paid for you, but you had to pay the core classes and buy books/supplies yourself. When I went to nursing school, a hospital here was doing this~ they only accepted 40 students per year~ you took a test and the top 40 got it. This was a private university, however, so the "rest" of the tuition was quite steep. You agreed to work for that hospital for 2 years after you graduated. However, when I graduated in 1989, another hospital "bought" my indebtedness, so I never actually had to work for the original hospital. That was back in the good ole days when a registered nurse was in hot demand!