Anew student observes that in his first class, the people sitting in the back row answer most of the questions, so in the next class he sits with the back row, concluding that this is where the smartest students sit. what is wrong with his reasoning?

The new student is  overgeneralizing the situation by concluding that because in the first class the people sitting in the back row answer most of the questions this means that the smartest students sit in the back.
The situation was only a coincidence, and the new student made fact of it.