Credits:
It depends on your school. While I was there, I had to take courses through the Disney College Program, and based upon my grades in those classes, they assigned me an internship credit at my school ( University of Wisconsin - Milwaukee ).
Some schools make it easier to get credit, just by giving you credits for being there, some require you to write a journal, and some do not offer credit. Unfortunately, this can only be answered by your Campus Rep team, or your school advisor for the program.
Semester -
Depends - I think if you can stand to be away from family and friends, the Fall/Fall advantage program is a good term. I did it, and missed Christmas with everyone I knew - but at the same time I think that really pulled some of the friendships I had down there together, stronger bonds. For Christmas, all we had were our friends there, work, and the parks. Plus, all the parks are decorated - If you've ever been there during Christmas, you'll know what I'm talking about. You'll also be at WDW for New Years Eve, which was fun. I was at Pleasure Island working, and got to sneak out to see the fireworks at midnight with my family that came to visit. If you choose the fall advantage program, don't forget the 4'th of July!
Spring - I have a friend down there right now. The advantage to this program is that you only miss one major holiday ( easter ). So if your worried about the holidays...
They treat the cast well during the Christmas season though. You WILL be busy, but they also have the Cast Holiday Celebration - which this past Holiday Celebration included a extra park ticket good for yourself and up to 3 guests, 50% a full service meal, free popcorn/drink/picture & photopass picture certificates, special rates on hotel rooms for family.
-Troy