We have been using Debian for four years now, on all of our servers, desktops and laptops. These are around fifty devices, used day by day by our students.
Our main educational goal, of course, is to show the students, what a community of freely connected volunteers is able to do. Secondly, they shall learn to deal with an alternative usability concept which is at least as easy to deal with as commercial standards. So, we have Gnome on all of our clients. And last but not least we use Debian because of its reliability.
On our servers, OpenLDAP is running for the administration of devices and users. A central part of the organisation of everyday learning is our internal website. And, up to now, the network is organised by Samba, to ensure seamless collaboration with another department.