When a device doesn’t check in to Jamf School for a period of time, the device will be marked as inactive. The time it takes till devices are marked as unmanaged is configurable at Organization -> Settings -> General -> Device Management.
Normally, just after checking in a device will be active again. It is known however that this isn’t always the case.
There can be some reasons why a device isn’t becoming managed again immediately, most of them are caused by not connecting to the network properly.
To troubleshoot this, most of the times it is enough to switch the Airplane mode On and Off.
If this action did not solve your problem, probably the KeyChain has become locked.
This will cause the issue that the Wi-Fi password is not transmitted to the Wi-Fi network anymore, causing a log in failure.
To unlock this status, you can just enter the passcode, of the device, switch the Airplane mode On and Off, check the internet connection and check if the device is managed again.
What you could also try is to tether the device to a Mac and share its connection, more info is found here: https://support.apple.com/kb/ph25327?locale=en_US
If these two steps did not help you out, we advise you to gather some information on the device and create a ticket for support in Jamf School Management System.