It was definitely for the best that Zacharias knew nothing of both her feelings and the kiss with Stan; he wouldn't really know what to make of either. He especially wouldn't be pleased if he ever discovered her kiss with Stan because Megan was his friend, and you just can't do. But as is, he was happily oblivious so he continued to smile at her.
"Mhm, clever of you," he responded with a soft nod. "That's why the put you in Slytherin, yeah? For your plotting abilities. Though I, of course, know the truth; I've seen you lurking about, I know how much you like stalking Hufflepuffs." It vaguely occurred to him that it had been awhile since he teased someone like that; he had recently been too absorbed in his own angst to let go and enjoy himself.