We've had some issues with both teachers accidentally connecting to the wrong TVs using InstaShare, and students who have found their way on the network connecting to the TVs. To help prevent this, you can put a password on your TV that whoever is trying to screenshare must know.
In order to set this up, first sign into your TV, then open InstaShare.
In the bottom right corner, click on the Settings icon.
Then tap the toggle switch next to Connect Code to turn the option on. If you'd like to set your own password, tap the pencil next to connect code, and you will be able to set it to whatever you'd like.
When you connect using InstaShare, it will ask you for the password that is in the Connect Code box. If you have the Connect Code Window option turned on, it will show your password on the screen at all times. I would recommend turning or leaving this option off to prevent students from connecting.