One thing comes to mind... Why is it that your admin is leaving security up to you...??? Anyway, there's a security tutorial at mamboforge.net that may help a bit.
I'm not sure how your school's users access your site, but (I'm going on memory here, so I'm not entirely sure

) if they are logging in and authenticating locally and are setup to be a member of a regonized *nix group... You may be able to set an Allow/Deny to bypass that group and force authentication on anyone else not a member.
If they aren't logging in locally, and are just accessing the site normally (just as an Internet user does), there might be no choice except double logins.
Just to be sure, there is quite a bit at apache.org on htaccess, and there are even quite a few htaccess forums that you can Google for it too.