A lot of schools don't have phone points in every classroom or even capacity on their pbx for that.
Surely an Edugeek member of your standing knows that some teachers are like Steven Seagal:
I <3 Lync... it's the dog's :)
AFAIK the Lync Standard CAL will soon be part of the core CAL pack too and the server license is cheap under subscription licensing.
BTW - I assumed that we were the only school with the register problem