Val,
Depending on the amount of security measures your school has, what you ask will be extremely difficult and in most cases requires a "court order" to gain anything substantial. There are a large number "Instant Messaging" (IM) programs available, not just two like the "kid" told you. "MSN Messenger", "AOL Instant Messenger", "Trillion", "ICQ" and a variety of others are extremely common. The most important thing here is they can ALL be used on almost ANY Internet Service Providers (ISP)network. A user can use AIM on the almost any ISP, "MSN Messenger" on any ISP, or "Trillion" on any ISP, etc. You get the picture.
Even if you do find the specific ISP or the e-mail address that a "screen name" was using, most ISP's will not "give up" the actual users name without a court order. The ISP MAY have the true name and address that is used to bill the user, but then again, if a credit card is used to pay the Internet access bill, all of the user information may be false.
There are lots of possiblities. Many schools have internet gateways with firewalls or access monitoring software. Did the firewall or access software record any of this? If you have a computer/network/security expert available, you might be able to find out some peritinent information. If your concerns are serious, (terrorist threats or activities, etc.) then I advise hiring one to help you or contacting the authorities.
Hope this helps.
Grif