chmap requires you have a prefix for the gametype if it's available in the map config... for example chmap ctf_capturecity will set the game mode to CTF and change to capturecity, where doing map capturecity will just change the map and leave whatever the last gamemode was on (DM by default)
yeah, they probably have something on their network switches as well, or all internal traffic is going through a filter of some sort. I wouldn't mess with that, you could get in trouble with your school's technology staff...