This issue is resolved by updating iOS.
Issue: Teachers receive error message when opening classroom app stating "Server session invalidated."
Impact: Teachers cannot use Apple Classroom
Solution: This issue is resolved in iOS 11.2.5.
At this time there is no feasible workaround.
Apple is aware of an issue with users of Apple classroom running iOS 11.2.2. The issue happens when devices have PAC (proxy) files installed. We use PAC files to use our web filter as a proxy. Apple recommends deleting the PAC files until a solution is available. We will not be deleting our PAC files, so therefore, the only workaround is to avoid updating to iOS 11.2.2 if possible. If this is not a possibility or too late, we will have to wait until Apple comes out with a new iOS release.