Unfortunately you can't run IE 6 on a Mac because it doesn;t exist.
(For all of those that say "use Firefox -- you're ignoring the fact that the website in question probably uses ActiveX and that is why it's not usable on earlier IE, Mac, or Firefox)
Your only option if this is the case (that the web site uses proprietary Microsoft technology and ignores Web standards) is to install Windows on your Mac with Bootcamp.
http://www.apple.com/macosx/features/bootcamp.html
Ask your school if they provide special pricing on Windows XP for students. While you're at it, complain about them programming a website that forces you to buy and install software you didn't want.
(IIRC, IETab for Firefox requires IE to be installed on the machine to run the IE engine... that's a non-starter here)
(cen - you would be correct if the website didn't *really* need IE and could be spoofed just by faking the user agent. But this is a math site, and probably is using an ActiveX script to run an on-screen equation editor or something, so Safari will fail on that.)