If you buy a machine with Windows 7 Professional, the EULA agreement (or what ever they call it nowadays) allows you to install XP Pro legally.
If you machine is a tier one machine (Dell / HP etc) then you might have driver issues.  Research the motherboard type and see if XP drivers are available or not.  If they are, load up XP and be happy.

Microsoft don't support XP any more. Most of the software written in the Education sector don't develop anything on XP. You are better off running the app in "XP Mode" within Windows 7. I don't have any issues with Windows 7, it is a VERY stable platform. My gripe is the microsoft AV - it is very poor.

Ironically, iPads are the preferred teaching tool these days. Microsoft finally release a stable platform, but then lose out to iPads and MACs.