the normal, home user, quick answer is Yes, just pull the phone line from the modem.
no internet but still full LAN services.
but in your case it sounds like the real question is "can I kill the internet for just my classroom" and to that the answer is No.
at least not without getting the IT Dept all in a tither if you start bringing in your own hardware, setting up wifi networks, reconfiguring printers etc., then putting everything back the way it was.
if you describe your setup in more detail that may help.