Well it depends on what you're talking about. If you go to a school that has both AA and A it may be a simple matter of which team you make at try outs.
What it seems like you are talking about is an entire program. That's a different story. Pretty much you need to establish your schools program in AA first, demonstrating that you have the structure, funding and players to run a Class A team. Then you submit your proposal and Chris/Rob look at all the schools that applied and give out bids based on what they see. Not every team that bids gets in (ex Northeastern last year) but you can still play AA events and bid again the following spring after Nationals. Another key is playing in A at Nationals the year you apply for a bid to A. Pretty much it would take at least 2 years to get into Class A as a program, maybe more depending on how quickly you get support from the school/number of people you have in the program.
Class A is really about sustainability and longevity. They want schools to enter that will be around and continue to be around in that format for years.
Hope that clarifies some.
Drexel University Grad
2009-2010 NCPA Class A National Champions