You are correct sir. Without patching Windows you can only have one user logged in at a time - so you're not violating that.
Frog doesn't do WOL AFAIK, but you could throw something together to turn machines on remotely without too much trouble - that way you wouldn't have to leave them running all the time.
Also, the main use-case seems to be teachers/staff who would usually be on site working from home during opening hours - so you're not actually using up any more power in the school as a result of the workstation being powered on in this case.
