If you are in the middle of class & a student asks you a question that you aren't prepared for and dunno the answer of, how will you deal with this situation?

Be perfectly honest and say: "Sorry I can't answer the question at the moment, but I wiil get back to you as soon as I can". You can't be expected to know everything but if you try to bluff it you could lose a good deal of credibility. Another option is to admit you don't know the answer and put the question to the rest of the class. You have shown that you are human and you have also shown that you would happily accept the fact that one of them might know something that you don't, it's a sign of your own humility and respect for them.