It grants the IT department authority over the devices. Restricting unauthorized changes like adding new accounts, adding new software, removing existing software, allows for tracking of the devices and sometimes remote wiping in case the device is stolen or lost and valuable data is on the device, among other things.
Less to do with cheating and more to do with control over the device since it’s the school’s property. Preventing cheating is an afterthought of MDM (mobile device management).