Site-specific User-Agent Safari

Solution 1:

Here’s a two-year-old topic on Quora asking the same question. No answer yet.

This leads me to believe there is a way to do what you want. Always changing the UA string using the menu is annoying, but at least it works.

The Opera web browser has a feature that does what you want. But of course, if you have to install a separate browser anyway, you may just as well install Firefox 3.6 and use that to access the site you’re talking about.

Solution 2:

It might be easier to just download Firefox 3.6 and use that for the school site and use Safari for everything else. You may also want to bother your school's IT so they remove the useless warning for compatible browsers.