This process will involve wiping device and administrative work in your Apple School Manager (ASM) account. So please sync the data properly and get ASM login info ready.
- Log into your ASM account and unassign the device with the Serial Number
- Wipe Device :
- Device > Device Management
- Select the device
- Action > Advance > Wipe Device
- Remove the Device from your Devices List :
- Device > Device Management
- Select the device
- Action > Advance > Delete Device