You can learn more about supervision here: https://support.apple.com/en-us/HT202837
If you purchase an iPad/Mac from an Apple Authorised Reseller (AAR) or Apple Authorised Education Specialist (AAES) (e.g. Compu b or Wriggle). They can add the device to your Apple School Manager (ASM) account and you can supervise the device over the air using a suitable MDM.
If you have purchased the device elsewhere (or it was donated to the school) or was not enrolled in ASM when purchased. You must manually enrol the device into Apple School Manager to supervise the iPad/Mac/iPhone over the air. You can manually enrol the device into your ASM using Apple Configurator.
Supervising 'over the air' gives so much control to the organisation without having to touch every single device.
To retain the the chain of custody an AAR/AAES can only enter the device that they have supplied into a school's ASM.
Devices that are in the school ASM are 'known to Apple' as owned/managed by the school and as a result can be unlocked (PIN/activation lock etc) (by AppleCare) for the school if needed.