You might try booting in Single User Mode, and then using the command line to setup a new admin account. The monitoring may be superior to just that, but it’s worth a shot.
There are two ways to boot in single user mode, depending on how updated the computer is:
Method 1 (the new method)
- Turn on your Mac, then immediately press and hold Command-R to start up from macOS Recovery.
- Select Disk Utility from the Utilities window, then click Continue.
- From the Disk Utility sidebar, select the volume that you're using, then choose File > Mount from the menu bar. (If the volume is already mounted, this option is dimmed.) Then enter your administrator password when prompted.
- Quit Disk Utility.
- Choose Terminal from the Utilities menu in the menu bar.
- You can now enter UNIX commands. When done, choose Apple () menu > Restart.
Method 2 (the old method)
- Turn on your Mac, then immediately press and hold Command-S to start up in single user mode.
- You can now enter UNIX commands. When done, type “reboot” to restart.
(Source: https://support.apple.com/en-us/HT201573)
Create New Admin Account
Once in single user mode, prepare to create a new admin account by performing the following commands (no quotes, make sure to include spaces):
- Mount the drive by typing “/sbin/mount -uw /” then enter.
- Remove the Apple Setup Done file by typing “rm /var/db/.AppleSetupDone” then enter.
- Reboot by typing “reboot” then enter.
- Upon reboot, the computer will walk you through the setup process as if the computer was new, having never been set up. Complete the setup process, creating a new admin account.
- Once the new admin account is set up, you should be able to erase the original admin account, including all files and preferences.
I haven’t tried this since the new protocol came out, so hopefully it still works. It’s a handy trick when buying used products. Let me know if it doesn’t work!
Its possible it was borrowed from the school ;-} I would go back to the person who ran the yard sale and get your money back as the easiest solution.
The other answer here, it's possible it was tossed and dug out from the trash. Give the school board a call with the systems S/N to check on it. They may offer a reward! And may even pay for the repairs!
Of course you could blindly get the system working by replacing the logic board but is this system worth it? High Sierra is the newest Apple supported macOS this system runs. Yes, you could get Mojave running but it won't get you all of the new features.
And besides you need to live with your self in the end. Supporting theft is not something I agree with.
door Dan