So let’s consider what those boys will have seen and heard. It is possible that after the teacher and the girl left the room, some of the pupils may have tried to peek through the windows to see the girl being slippered in that corridor. But let’s discount that and consider only the very minimum they will have seen and heard.
They will have heard the teacher shout at the girl to “stand up and come here”. Then they will have seen the teacher take out his/her slipper. At that point the atmosphere would have been one of tension and suspense. Boys would have been looking at the girl and noticing the fear on her face as she walked to the front of the class and as she looked at the slipper in the teacher’s hand.