[Adam Krajewski]

The big SCotUS case in this matter is New Jersey v. T.L.O. (1985). The key idea in the decision is that school officials may conduct searches under "reasonable suspicion", which is considerably less than the "probable cause" needed by police in other situations.

Random locker searches are allowed due to some slick maneuvering in that they are claimed to prevent rules violations rather than catch offenders. In Zamora v. Pomeroy, it was deemed legal for drug sniffing dogs to sniff around lockers and cars to not be considered a "search". However, if the dog detected anything, that provided the necessary "reasonable suspicion" for a search.

As far as the AIM sniffer, there are several different ways to use public key encryption over AIM, which would solve your immediate privacy concerns.