Students with any knowledge of IRC can always just use /server to access other networks. If you really want to use it and have it "locked" to only your private network, then I would suggest using a quality firewall that allows you to only allow IRC connections on one network/server. Then, no matter what a student does, he/she cannot connect to other networks.
Beyond that, IRC in a school setting should be supervised. I've done IRC with students myself, but rather than lock it down, I just made sure that I watched what they were doing. Of course, this requires that the teacher(s) know IRC and computers really well or students who may know more can always "fool" the teacher.
