This is really cool, since I'm currently working on a way to transmit extremely small amounts of data (~30 bits) in a situation where you cannot use any RF waves.
Of course, it's banned here but the simplest thing (for the person taking the test) would be to take a short amount of time - or a long amount of time - to fill in each bubble. A short scribble = false, a long scribble = true. To thwart the proctors you could swap the keys every other question, such that for question 1 short=false, long=true; question 2 short=true, long=false, etc etc.
Of course, for any of these answers the proctors could trivially catch you cheating. The best way to prevent this would be applying a simple one-time pad to them - exchange a small number beforehand and use that.
Of course, it's banned here but the simplest thing (for the person taking the test) would be to take a short amount of time - or a long amount of time - to fill in each bubble. A short scribble = false, a long scribble = true. To thwart the proctors you could swap the keys every other question, such that for question 1 short=false, long=true; question 2 short=true, long=false, etc etc.
Of course, for any of these answers the proctors could trivially catch you cheating. The best way to prevent this would be applying a simple one-time pad to them - exchange a small number beforehand and use that.