Most recent distros install firewalls that block off most ports. If you are trying to connect but get an error message like "Connection refused" this might be the case. Telnet might also be denied with TCPWrappers. What distro you're using will help us in pointing you in the right direction. Another possibiltiy would be that telnet is blocked by a firewall that your school's network administrator setup.
BTW, Why wont SSH work. The is a free client called Putty
which will allow a Windows machine to connect to a machine running an SSH server. I would strongly recomend that you talk to your teacher and try to get him/her to use SSH instead. If your school uses hubs instead of switches anybody wont have much problem getting you telnet password or watching your telnet session.