Well for classrooms that have 30 clients, I'd say you can cover more with 1 AP.
For 600 you need to start asking your self, is everybody going to access it at the same time. I've had a 2000 user deployment at a conference hall. I used 3 7372s, but there were only 300 users at one time connected. That's under 20%, but that was a conference. One 7982 can take 300 users but it depends also on what are users doing with their clients? Will they all require a connection at the same time for some video streaming or just some browsing and e-mail and stuff?
Be careful, as you can get into more problems with using too many APs at one location than you think you'll solve. It's so many things to consider.