The best way to factory reset a DEP device ist to do the following in Control Tower:

1. Settings > System > enable  Re-enroll Apple devices into the previous device group > Save
2. Devices > click on the name of the device group > put a check in the box next to the iPads you wish to factory reset > Select the Erase Device button at the top of the list.
1. enable Clear activation Lock
2. enable Delete Devices from Control Tower 

Once the device factory resets follow the prompts and the device will enroll back into Control Tower.