Label it with a paint pen, and perhaps a magnetic strip.
Velcro it up out of reach.
Put it on top of the projector.
Screw it down using galvanized strapping (from the plumbing department, I could probably give you some).
Put it in a cupboard or display case that has a transparent or translucent door.
Cut a custom CNC wood case that can be bolted down.
Unfortunately the Apple TV is small and portable enough that students may be tempted to walk away with it. Hopefully something like one of these ideas can help prevent that.