The reason that we want seperate ssids is because we consistantly have our passphrases compromised by teachers/staff sharing them with students or sharing with unwanted users. When this happens we have to change passphrases and re-add them to each teacher laptop, and each one of our portable labs which is a huge damper on our time. We are the only people who are supposed to know the passphrase and add users accordingly.
The other reason is because all of our student computers are locked down with a product called smart control and to change each one of those computers would require us to unlock everyone of them and reload the passphrase, probably 500 computers or so. So it would be nice to keep our current ssid configuration.