I can see the cost aspect for definite there with your heavy / total use of Open Source.
Interested if you use MS AD for your workplace / school computers?
Myself and a few other started an outsourced / Hosted Services company a few months ago, after 6 months of deployment building the platform (Highly redundant w/ support for upto 10,000 Concurrent users) for the three MS UM components (Exchange, OCS, Sharepoint) inc SIP VoIP calling support.
I was curious as to what the motions are in the schools sector,
we hand the control of all your users and accounts to you, and can integrate your logons with your own AD.