I worked as an EMT-B for a private company for a year, while going to school full time (I think I took around 14 hrs both semester, but 2 hours were research). My work shifts were 24 hrs long and I usually pulled 2-3 shifts a week. I had to schedual my classes on Mondays, Wednesdays and Fridays...Normally, I worked from 7 in the morning until 7 the next morning, and headed straight to my 8:00 class. Usually wasnt too bad, unless i ended up having one of those nights where we were called out every 15 mins, preventing us from getting any sleep really. I also made sure all my professors knew what I was doing because there was a possibility, and it happened several times during the semester, where if the weather was bad or something and the transport helicopters couldnt fly, I would be sent out on a long distance transport in the middle of the night...generally 8-16 hour trips and just couldnt make it back in time for my classes...
Interestingly, I took some of my more difficult classes that year, and ended up bringing my GPA up quite a bit...
You can do a ton of studying on the long distance transports when your patient falls asleep and to echo what Markobenin stated above, you have to become extremely good with managing your time because you never can really plan to do so many hours of studying in a day as you dont know when your going to get called out and how busy the calls for the day will be.
Its also a good thing to make good friends with someone on another shift so you can swap shifts with them...especially people who have a family - the ones I worked with were very happy to swap a weekday shift for a weekend shift whenever I have an exam coming up.