Chasidut על תהילים 2:11

It is not that he should be light-headed in front of the student nor disgrace himself in [the child's] eyes, God forbid. Just the opposite, the teacher should make efforts that the student see him as a man of stature - yet still also as a good and easy-going man. Even when he shows them something humorous, the student should tremble. And even when he puts fear into him, there should be a pinch of joy mixed in. "And rejoice with trembling" (Psalms 2:11). [The child] should tremble in front of him differently than he does in front of an irritable, angry or violent man; but rather with fear that comes from honor, like from one of the lofty ones. If the rabbi is similar to an angel of the Lord of hosts when he gets angry at [the child], the student will not hate him on its account. And when he shows him a pleasant face, he will rejoice and love him and fear him even more.