Hi Mikey,
please note I have not used this personally, this is information based on what I know from reading about these programs
So Apple School Manager using an MDM server is intended to deploy and manage a wide deployment of Apple devices. This can also include DEP(Device Enrollment Program). If this is all correct, even factory reset will still point the AppleTV box at the MDM server.
There is nothing that can be done to change this to the best of my knowledge. Think of itAs activation lock. Until that lock is removed the device cannot activate. Being on the DEP list, it cannot do anything but get pointed to School Manager MDM server. In other words, that AppleTV must first be removed from DEP (and maybe even the MDM server).
This is to the best of my knowledge, however, maybe another user can give you better information.