Blocking minecraft

I was asked by a principle at a school to host a minecraft server for the students to play together, the only problem is we do not want the students to be able to connect to external servers but minecraft requires Internet access to login and play on the server I set up. Is there a way to block minecraft accessing the external servers but not minecraft login services?

Thanks

Yep, you don't want to run a normal Minecraft server, you want to go through MinecraftEdu and buy your licenses and server there. The server they sell contains a system to allow login without using Mojang's auth servers, and the end user licenses are much cheaper than normal.

Plus, its all legit and authorised by Mojang.