Let’s assume all students do remember which account they used. Their accounts are not generating any traffic. Why would it be bad if their device is still powered on and transmitting data?
Or are you saying that the students are no longer using the devices, and you know which device it is, and that you want to register that device again (like to use it with other students) but cannot do that as TTN Console is giving you some error message while trying to register it?
Anyway: given a DevAddr you cannot tell much.
(Well, if an OTAA device is actively registered in both networks at the same time, then both networks might send an OTAA Join Accept if both happen to receive its Join Request. But while registering it, like by using some website such as TTN Console, that website won’t know if it’s already registered at some other network. Of course, “i cant register” is not a very descriptive problem description, so maybe you’re referring to something else altogether…)