The Debian distribution is the largest distribution used in this university-like school, including more than 20 laboratories. The best and the original use of Debian is the network one: routers (under ATM and Gigabit Ethernet technologies, with dynamic routing inside and outside campus), firewall (stateful filtering on each router), VPN gateway (based on the OpenVPN project). Debian is also used in all DSI (Direction of Information Systems) Linux servers, databases (MySQL, PostgreSQL, Oracle), network services (DNS, proxy, LDAP, news, etc.) and workstations in free access rooms. Finally, all current Debian distributions have been installed on the meso-calculator (Nec TX7 with 32 Itanium2 processors) without touching to the original and only supported distribution from Red Hat.
Also, for years the ENS-Cachan has hosted an official FTP mirror of the Debian distribution. The students organize Debian install parties.
The choice of the Debian distribution is historical (the most used Linux distribution amongst the system administrators), philosophical (independence from commercial contraints), and pragmatical (Debian is easier to administrate than to install ;-) ).