Turn of the Screensaver: Nothing better than having students read something off the screen when suddenly they are looking at a zebra, walrus, flower, etc... Not helpful. Under settings go to screen saver and "start after" to never.
Get a password: I'll never forget the day I was teaching when suddenly a football game showed up on my screen in the middle class. This was definitely not in my lesson plan. I knew I needed to do something to make sure something worse than a football game didn't show up on the screen. Under the settings menu go to General and choose Restrictions to setup up a password. Once you enter it once you rarely have to type it in again. If you have student devices and want them to use the AppleTV to show their work on the screen you can enter the password and it should stick to the device.
Customize the Home Screen: Again, unwanted distractions are everywhere and if it's on your screen you're responsible for students seeing it. One of the things AppleTV does is promote the latest and greatest movies right across the top of the main menu screen. Right now one of the big hits out there is the movie Sex Tape. Not a great visual for keeping kids focused (especially middle school boys). There is an easy fix for this, just go to Settings and Main Menu. You can turn off any of the icons on the screen that you don't want to see. Here is a pic of my main menu screen.
Apparently I left out an instruction that actually gets rid of the movie ads. The editing the main menu gets rid of most of the icons but not the movies. To get rid of the movie ads you need to go into Settings, General and then Restrictions. Set Purchases and Rentals to hide and that should get rid of the ads.
Hope these few tips help. If you've got some I haven't thought of or mentioned please reply in the comments. Happy teaching!!!