It wouldn't exactly be hard to have the signs say this is only 1st class if it is at the front of the train, not if it is at the back, or whatever the situation is.
This wouldn't even be foolproof in my experience. I've lost count of the number of times people have boarded a train before spending considerable time working out between themselves which direction the train is going despite being at the bufferstops. Then they somehow still get it wrong and switch seats when the train departs.
If they cannot work this out then i'm unconvinced they can figure out which is front or back.