Can I use a school-managed Chromebook after installing another operating system?

A school-managed Chromebook isn’t normal for a personal purchase. Contact the seller and ask for a refund, or ask the school’s IT department whether they can officially deprovision it. Without that, there probably isn’t a legitimate way to use it normally.

Most Chromebooks aren’t well suited to Windows, and Linux support varies by model. Management may also prevent you from enabling developer mode or changing the boot setup. A Chromebook normally can’t work as a monitor either, because it doesn’t have a video input. I’d return it rather than trying to modify it.

The best option is to request a refund. The organization that manages it needs to remove the device from its system, usually using the Chromebook’s serial number. Installing another operating system generally won’t get around that management, and a managed device being sold this way may have been improperly sold or stolen.