I wonder if you have some more recent Adobe software on the machines which users are signing into, which may be breaking the serialisation? This would even happen if they sign in using an Adobe ID on the website.
The only advice in that case would be to ask students not to sign in.
Other things to check for are that there are no entries in the hosts file which are causing problems. This can be cleaned up using the CC Cleaner Tool, in fact I would advise uninstalling, running this tool and reinstalling on at least one machine to test.
Use the Creative Cloud Cleaner Tool to solve installation problems
I would also check that your firewall is not blocking endpoints required for activation.
https://helpx.adobe.com/content/dam/help/attachments/Creative_Cloud_for_enterprise_Service_Endpoints.pdf