If you’re an administrator and want to set up supervision, use Apple Deployment Program, Configurator 2, or Apple School Manager to set up your devices.
Apple Deployment Program - https://help.apple.com/deployment/business/
Configurator 2 - https://support.apple.com/guide/apple-configurator-2/welcome/mac
Apple School Manager - http://help.apple.com/schoolmanager/
Ideally the school should have been using the above services. If not, you will need to get somebody recognized as an official of the other school to take the devices along with the original receipt to an Apple Store and request they wipe them.
Find My iPhone Activation Lock - This is a security feature, in part to deter re-selling of stolen devices.
http://support.apple.com/HT201365
Turn off Find My iPhone Activation Lock (formerly: Activation Lock: Removing a device from a previous owner’s account) - An activation lock can only be removed using the log-in information for the account that applied the lock.
https://support.apple.com/HT201441
Get the owner of the account to remove the lock or return the device for a refund. The device is otherwise useless to you.
If the owner has not provided contact information as a message showing in the login screen, there is no way to contact the owner unless the person who gave you the device knows them.