you can link to one of the below sites - most likely not all will be blocked by schools/offices (most schools/offices allow access to secure http sites by bypassing the firewall/proxy; meebo has https access)
meebo -
https://meebo.com/
WebAIM -
http://webaim.net/
eBuddy -
http://www.ebuddy.com/#AOL
AIM Express -
http://aimexpress.aim.com
QuickBuddy -
http://toc.oscar.aol.com/tic.html
ILoveIM -
http://iloveim.com/
Kool IM -
http://koolim.com/
you can also chose, if you are skilled, to run your own Web AIM-like service for your members.
http://webaim.net/developer/index.html has related information.