It seems Apple are making things a lot easier if you are trying to integrate Apple TV into your secure "Enterprise" style school network. A very common authentication standard used for most school wireless networks is called 802.1X. Placing a device like an Apple TV (where you cannot specify credentials such as 802.1X) has, until now been very hard.
Added to that your school likely runs all internet traffic through a proxy. Now you can add the proxy details to your Apple TV too.
The latest Apple TV software update (5.1) now allows the option of creating a Configuration Profile. This profile needs to be made inside the Apple Configurator and then installed onto the Apple Tv with a micro USB cable.
For detailed instructions:
Now it should be a reality that you can use Airplay on your school network, hooray!