"At this point, all the web browsers pretty much work the same way."
That's what I was told in 2008, when I was trying to sell the idea that organizations should have more than just Internet Explorer on their standard desktop image. I didn't bother pushing the point -- my elevator pitch wasn't prepared for beliefs that far from the truth.
Soon after that, Chrome was released and literally revolutionized the web browser landscape. If you haven't ever read it, you seriously need to sit down and go through the entire Google Chrome comicbook, so you can appreciate what a huge change it represented. Almost three years later, Internet Explorer is just beginning to catch up to what Chrome was doing in Fall of 2008!
What's wrong with Internet Explorer?
IE's web standards support is years behind all other major web browsers. This doesn't just mean that your teachers won't see shadows and design elements (although that's true, too). I'm talking about core, fundamental features of the web that are supported in current versions of Chrome and Firefox that aren't yet supported in IE 9 or even in the current previews of IE 10. (References: charts comparing many browser versions at once, or compare two specific browser versions in detail)
IE is slower than other browsers. If you give teachers the option of a faster browser, you could save money by stretching the life of your hardware. (Reference: typical browser test results) Additionally, because IE loads pages slower, you have to compensate by lowering the quality of those pages, and the quality of the tools you offer to teachers.
If you only have IE, then IE problems are always show-stoppers. Teachers can't easily work around web tool problems that only affect IE because they don't have other browsers installed.
Those are solid reasons on their own, but they're all based on the current version, IE9. If your computers use Windows XP, you can't even install IE9, it only supports Windows 7! These problems, then, are all multiplied because you are literally using a browser (IE8) that was behind the times when it was released over two years ago.
If you have to decide on one additional web browser (and you don't), you should offer Chrome.
Firefox is more commonly known, but Chrome really is the leader. It is faster and more secure/stable. Chrome already has 20% market share and is on track to have more users than IE or Firefox in less than 2 years. (Reference: Pingdom blogs about Google Chrome)
On a practical note, Chrome also seems to have stronger support for group management than Firefox, although there are some Firefox options out there.