Why did Evans not take off his hat when Jackson ordered him to do so?

Evans did not take off his hat when Jackson ordered him to do so because he had a plan in his mind. Evans had long wavy hair. He had decided to crop them short so as to impersonate McLeery. But this would have been evident if he had taken off his bobble hat. So, he took permission from Jackson to keep his hat on while appearing for the exam on the pretext that it was his lucky charm.