Outside the school, 3 students, William and others, are harassing Margot because she has major differences from the rest; mostly behavior wise at that. So then they proceed to lock her into a closet simply for the "jokes".
In this scene, the teacher had asked the students if everyone was present in the classroom, not taking the time to check each individual student, which will lead to an issue soon.
Margot watches the sun rise through the window, but cannot actually go outside to feel the suns radiance. She begins to cry.