Make them use a passcode first time, no mistakes. Assuming your teachers don't move from room to room, the last connected one should be on top of their list

We try to stay out of it as they are professionals.

What @larry_barrett says. I find this to be the easiest way to guarantee they don't end up on the wrong one.

However, I do have to point out that there are numerous ways to limit what AppleTVs someone sees. From iBeacons, IP/Building detection within JAMF, Other JAMF Policies, Wireless Access Control (We do this using ClearPass). There are options.