They get you because you are in training mode, and they are in sparring mode. Your subconscious is locked on one specific scenario you are about to teach, and you must demonstrate that.
You have whole load of things to think about:
1- Not hurting the student unless you have a favorite demo guy that likes to take hits.
2- A tight schedule to run and structured training to complete.
3-By the way your inexperienced student stands you are hoping he will execute something that looks like the required specific stab or slash you've asked for.
4- In fact if he is in playing mood he will make you look bad because you are looking to finish a straight forward demo using a specific technique and not to respond to changes. He or she on the other hand are not looking for a committed attack necessarily. The easiest way to get you would be to do something you do not expect. They will feint(a deceptive move showing you[re going to do one thing while doing another) for sure in one way or another) for sure.
5- with a knife which is a long extension, it is hard to block and counter attack simultaneously and many times you need to close the gap before you can attack. Of course if the opponent retracts you follow up with a kick.
So you can be dramatic punch the student in the face, yell at him for not doing what he is told, and some students will think you are a jerk and very mean. If he did not get you with the rubber knife, at least no one will think your techniques are not good. But many times you deal with students you need to teach quickly and they are not that disciplined yet.
Another way to handle this, is simply say. This is sparring mode, as you are trying different things and all I am trying to do is teach you one principle out of many. If you want to spar, that is fine. If the student is suitable to take few slaps and light striking you can let him try few times to prove that you know what you are doing, and then ask him to do a specific attack for a specific demo.
Instead of having fearful disciplined students that are afraid of punishment from their master and will hardly ever reach his abilities, you get a greater learning experience.