We use Debian on two gateways that connect the school and two other buildings on the campus to the Internet since 2008. Furthermore, we use some Debian installations in VirtualBox to teach our students basic system administration. They all run Debian Squeeze. On the gateways we run a DHCP and DNS server, Squid and an iptables firewall.
We chose Debian because of its stability and ease of maintainability. What is especially nice for us (as we have only limited resources), is the easy upgrade path Debian provides from one release to another, which prevents us from having to reinstall every two years.