A brilliant explanation of CALs
Understanding Microsoft Client Access Licenses (CALs)
I have added a couple of links from The Licensing FAQ which explain that schools agreement customers do not required external connector licenses, Client Access License Coverage for Educational Institutions and that Student CALs cover parents/guardians. Licensing Parental Access.doc so that you have all the information you need.
I'll bookmark this for the next time I'm having one of those discussions with SLT