Ok, my school are kinda the same, cant install programs, not got msn etc etc.
1. Type emessenger in google, and find the one that says emessenger login, NOT emessenger, just emessenge login, this might not be blocked, heres the link
2. Try iloveim. New web messager. http://www.iloveim.com/
Alot better than the other one, but sometimes doesnt deliver all messages. Try both of these and please let me know. Ive installed remote control programs on 30 or more pc's at school, so I can access my school account from home.