heh, you sound like one of the guys in my class. He was prior Army, got a job in security and is now in class with me for NLO (AO). He is no where near close to having a degree either.
Talk to your HR rep to find out what you need to wear. Casual means business casual, not "casual" haha. As Murray stated, find out if your poss and interview will be on the same day, some are, some arent. button up shirt, nice tie, and dress pants is a sure bet in any situation.