For the exact same need and for security testing (of vulnerability of different operating systems), I evaluated many virtualization environments.
I finally adopted VirtualBox because it was able to run many Windows versions, many Linux versions without too many usability and configuration problems. From a security standpoint and from a teacher standpoint the ease of use is a wining point.
As a security engineer I test operating systems under condition where they die (most notably weapons against Windows), and VirtualBox survived.
For a teacher, here are 2 more pieces of advice:
- To correctly help your students working with Mac, it would be easier for you to work with the same environment.
- Software which are running on different operating systems (MacOS X and Linux) are usually of a better software quality and stability and better suited to make student work on them.
VirtualBox will let you save a virtual operating state image which could contain softwares already suitably configured for your students to save a lot of time.