I use all three, depending on which operating system I'm using. When I'm using Windows, I mostly use Internet Explorer. If there are some certain sites that don't work on that browser, then I switch to Firefox. Whenever I'm using a Mac, I use Safari.
I've been in 5 schools so far, and only one of then had 1 iMac.
The school I'm currently on, mostly has either Windows XP, or Windows Server 2008 R2 installed, and sometimes Ubuntu (and a Hackintosh of a teacher).
If there are Macs at school, then they're student's own MacBooks.
To make a long story short: Not everyone has Macs at schools here.