I believe it uses IAX MD5 challenge/response authentication. So a brute force or dictionary attack could work, and also a man-in-the middle attack. It's much better then plaintext, not nearly as good as RSA. IAX MD5 is pretty common, and at worst with a hacked IAX connection someone could connect to your node and transmit on your radio.
If no other ports or services are on the box it shouldn't cause a network security issue. Now leave SSH running with a simple dictionary attackable password and it's game over. If you want SSH running, use a RSA key and disable PasswordAuthentication. Also you should limit ssh access to a handful (ideally one) user. Physical access to the machine should also be secured, CentOS in single user mode will allow anyone to log into it, change whatever, and log out... but that goes for any machine on the network.
If you're really concerned get shorewall or some other firewall running on it. Close all ports other than SSH and IAX. Then concentrate on the students, because they're almost certainly a much larger security hole :)