I have had this happen as well.  I also play on the Steam version.  Seems only to happen in the early years before I have enough amenities to cover the situation.  I have had it happen at the one where you start the spy school and also the one where you start musicality.  Not sure how to fix it.  But they never seem to actually drop out unless I expel them.  I have not had intruders or double agents freeze up like that.  Students unlike teachers, I am not sure if there is a like a threshold where they will just go fix an issue if it gets too low.  The frozen ones seem to be zero on energy, as well as nearly every other measure.

Hi Mutley,

Sorry to hear you are experiencing this bug with students getting stuck. I completely understand the frustration. 

We're doing everything we can to fix this issue, but the difficulty is that differing factors can cause the NPCs to get stuck.

At this time the only sure fire way to remove a stuck student is to restart the level from the beginning.

Some players have found that selecting the student and forcing them to do something else (such as go to private tuition) can help, but this is not 100% effective as you have found. 

Another potential workaround is to place a room over a student as this makes them jump back to a hallway, potentially helping them find their path again.

We will continue to work on this issue, so if you have any additional information that might help, please let me know. 

Thanks for your patience,

Chizel