I have had the same problem with my Chromebook. Here's what I did to take it off (before you start, remove any flash drives or memory devices):
Press esc + refresh + power (note: refresh is the 4th key from the left on the Chromebook. It should be the swirly arrow.)
Press ctrl + d.
Press space (the spacebar) or on some Chromebooks enter (the enter key) Note: This will put you in developer mode, let your Chromebook load up everything and do NOT turn it off yourself. Your Chromebook will restart and then start initiating dev mode, that will put you off of the domain.
Important: If your Chromebook says that dev mode has been blocked by the admin follow these steps:
If you see that dev mode is blocked, do NOT turn off your Chromebook, it will take you back to register your Chromebook again do NOT go through this process, instead, try pressing esc + refresh + power again.
See AlsoEnable TPM 2.0 on your PCPress ctrl + d.
Press space (the spacebar) or enter (enter).
Repeat this process continually until it works.
NOTE: Your Chromebook will restart again and hopefully load everything and boot you into dev mode, this means that you will no longer be registered under that school's domain and you can get full functionality out of your Chromebook.
Hopefully, this helps someone.