Authority in a classroom comes from presence and fairness more than from toughness. Good classroom managers have what researchers call with-it-ness: they notice what is happening across the room and address small things before they grow. That awareness, paired with a calm and steady manner, signals that the class is in good hands.

Be firm and consistent without being harsh. Students cooperate with an instructor who is fair, predictable, and clearly in command of the room, and they push back against one who is arbitrary or hostile. You do not need to dominate the room. You need to be present in it, aware, and fair, and the rest tends to follow.