People in all branches of the military get to go to school on their off time...it takes longer to finish a degree. Some of the schools don't have the classes you need or the programs you want. Sometimes you have to quit mid term and deploy somewhere. You also have to miss some classes to stand duty and what ever else is going on.
The colleges that you can go to will depend on where you are stationed. You can go on-line or you can go to a training room and some colleges come to you. If you are stationed in different states you can go to the colleges there. Some schools waive out of state tuition for military and you pay in state fees. It is up to the school.
If you have any way of finishing your degree before you enter the military, then that is what you really should do. If you know what you are majoring in and already have most of your course work done, you really would be better off to finish.
You can still enlist or apply for a commission after you get your degree. But if you are just tired of school and need the break go ahead and join. You can get your degree, but it is much harder and you may have to put it off for a while.