Are you married to CentOS and Webmin? There's a number of good step-by-step tutorials for Squid on other distro's, like Debian:
http://www.howtoforge.com/debian_ebox
Squid is really not that hard to configure outside of Webmin--it's simply a matter of changing some text files. The files themselves are well documented, too. If you need to stick with CentOS, I strongly encourage you to drop the Webmin requirement and look into the configuration files.
Finally, since you're doing this for a school, you may wish to look at Dans Guardian after you get Squid running.
Chris