There is a big difference between questions and second guessing. Questions can help you understand the motion, or concpets of that particular part of the training. Questioning what is being taught is a little different, some people just can't get it in their head that the instructor knows more than them, and have to question everythig that is said to them. These Nit Pickers can often break up the flow of a class, and be a problem for the rest of the class. In the past I have taken that student aside and explain to them that if they would wait until the lesson is complete they might learn something, and that they are also slowing down the others students progression. Some take it the comments I make and use them to help themselves others have tried to lash out. If they have a problem with the material being taught I refer them to a competitors school, let them have the head ache. If the have a problem with my teaching style I ask what the problem is, if it just something along the lies that they want everything handed to them with out having to work to learn, I challenge them to try it my way for a month, if they won't try it, I refer them to a competitor.
If a student challenges my authority in class, I remove them from that class. If they do it a second time, I show them the door, and send them off to a competitor.
If oyu are running a school you have to think about every person in the class, not just one troublesome student. You have to look at whether this one person's constant questions and out bursts are driving people away, I would rather my competitor have that headache so I can concentrate on the students that wants to learn.