How can I add another account on my chromebook when my school blocked?

My chromebook has an admin on it, how do I remove it?

See here. This topic has been done to death and back. The way the management works on Chromebooks means the admin needs to remove the restriction (spoiler: THEY WON'T).

I can wipe a Windows computer in seconds with management with a NVMe/ATA Secure Erase in seconds, reinstall and its gone, which is why any smart sysadmin ties it with tools like Absolute Computrace which has BIOS persistence. This "wipe it and run" approach doesn't work on Chromebooks since the S/N is tied to the management console, not the Windows install. This (and cost) is why schools largely issue Chromebooks to students now.

The "erase and reinstall" approach makes bypassing it inherently useless for network mischief, but it makes it easy to wipe it out for secondhand buyers when someone left a managed copy of Windows on the machine. With a Chromebook, I either need to contact the admin, or strip it for parts.
Even if the machine is not tied to a MS AD account for management, you usually need to wipe the OS and start over, even if it was "done" because some places like Goodwill Computer Works tend to bypass it and it eventually locks up beyond being able to access the PC anymore like after a freaking Windows 10 21H2 update (and you regret just not getting it done). With 10 and 11 being so easy, it's better to do it yourself, or pay someone you know a few bucks/beer to wipe it out and reinstall Windows for you when they have a chance, or bring it to them when they can do it.

I've trusted "clean" installs before from places like Goodwill Computer works when I got HW for cheap (with planned upgrades) and it always ends in disaster; be it unknown Bitlocker keys and unknown users which displace my user account beyond recovery, where it's now involuntary.

the only thing you can do is modify the permissions of your chromebook from the administrator's device