How can I reset my used iMac's to factory settings

Those computers that were not properly prepared for transfer of ownership will never be capable of being set up and personalized for use by a new owner. They may boot and be useable, but they may never be properly associated with the new owner's Apple ID, including access to App Store purchases, application updates and subscription and purchase history. They cannot be configured to properly associate with Find My and other security and other Apple devices. They may not be capable of being upgraded fully or successfully. Eventually this will become a hindrance to the user.

They will always be associated with, and access limited by, the identification of the previous owner. There is no workaround other than as described in the guidance I linked in my previous post.

This is a liability to anyone who buys any of these thinking they are unencumbered and properly released. To sell these off without disclosing that they are still associate with another's credentials would be inappropriate and I hope you wouldn't do that. When you do sell them, be honest and sell them "as is" or "for parts only".

If you really wish to remove all the admin security properly so they can be fully useful, you'll have to approach the previous owner and convince their IT to do things right.

This guidance must be followed by the previous owners of those Macs. What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

Otherwise they are nothing more than door stops.

I hate to be the bearer of bad news but D.I. Johnson is 100% correct. You should never have bought them as you are likely going to have some pretty unhappy buyers. The best you can do is(assuming returning them is not an option) is to fully disclose to a buyer.

Prior to buying a used Mac you should ALWAYS ask and have the seller prove that the computers were prepared per Apple's directions in What to do before you sell, give away, or trade in your Mac otherwise at best the computer will be associated with the original owner.

Sorry you were not aware of this in advance, for now consider it a painful and possibly expensive lesson. One thing you might want to do is to sell the computers for their parts.

Also, remember to erase them as the third party apps they include almost never include new buyers. So the computer's new owner will essentially be buying stolen software.

Good luck!

Macs made in the last few years – whether Apple-Silicon-based Macs, or Intel-based Macs with T2 security chips – support Activation Lock. There is no substitute for the previous owner preparing them properly for resale.

Even with older Macs, which were more like traditional systems in allowing you to reset a system provided that you could get physical access to the console, you could run into issues if

• The Macs had firmware passwords set, or
• The Macs were under MDM management (which it sounds like all of these may be)

Go back to the seller and ask them to properly prepare these Macs for a new owner. Or to refund your money. If it is impossible to get the help of the seller / school district, and they won't give you a refund either, I believe that Mr. Johnson would be correct in saying that you have a collection of door stops.