First Case, Second Case

While the teacher is faced towards the blackboard, one of the students makes some funny noises. The teacher can't find the person who did that, so he decides to dismiss a group of students for one week or until one of them tells him who was the one that made the noises. In the first case after some days, one of the dismissed students tells teacher the truth. In the second case, no one tells teacher the truth and the period of punishment finishes.