some people from the community have already replied you, I would like to say from Zentyal staff side that we have successfully deployed Zentyal in big schools and universities, for both gateway and UTM roles, doing all the management of the Internet connection: firewall, HTTP proxy, balancing and QoS, etc. and also as an office server with users&groups management, domain controller, file and printers sharing, etc.
When doing such deployments for many users, optimizing performance and having vendor support to address on due time any issue is very important. I encourage you to have a look at our success stories: http://www.zentyal.com/en/references/our-customers/
and don't hesitate to contact us if you have any question on info AT zentyal.com.