If it's the school's Chromebook, they own it and control every aspect of it. Get your own Chromebook and do what you want.
If it's your own Chromebook, but you've signed in with a school-managed account, the school has taken over your entire Chromebook. Yes, they can do that, since it's the only way to manage the account without the account being exposed to hacking from another account. You can get your Chromebook back by restoring your Chromebook, or if they have that blocked, recovering your Chromebook from a power-off state, signing in with your personal account, and then NOT ever using the school account on your Chromebook.
Managed Chromebooks, and personal Chromebooks which have signed in to a managed account, are no longer in your control, and there is no way to bypass the management.