How do I setup a pentest lab for a univeristy?

Question

I am working with a local univeristy which wants to teach students security. The modules involved using tools like Nmap and Metasploit on Windows and Linux boxes. However, as Windows has licensing issues the school does not want to hand over VMs to the students.

So basically here is what they need:

1. They have multiple licenses available for windows and want me to create VMs
2. A student can login to the lab from school/home over a VPN
3. As soon as he logs in he can start / stop VMs which he would like to practice on
4. A new instance of a VM image is created every time a request is made in (3) from a common image which is identical for all students
5. When the student logs off the VPN then the instance is destroyed
6. Multiple students may be logged in simultaneously and be working

Basically, this will allow them to reuse the same set of VMs for 50 students.

I am looking for open source solutions so the univeristy does not have to buy licenses.

QUestions:

1. What VPN solution should I use?
2. How do I create a virutalization setup where the same "physical image" can be loaded simultaneously for multiple students?
3. Any software available to automate the above tasks?
4. Can I use commodity hardware to achieve the above?

Appreciate any help you can provide!