Something that took place in school

Something weird happened at school. First lets start from the start of school. As usual we had literature for the first to periods, mother tongue, PE-theory, Maths and History. The subjects were not boring. We got a lot of homework as usual, suffering during the weekend. Wait let me continue my story. We were studying through history period for about 40 minutes when the whole classroom was dimly filled with smoke. At first nobody noticed it. After a while people started to notice it the smoke and were distracted from the lesson. Since I was sitting relatively near to the window, I was one of the first people to notice the smoke. By the time the teacher noticed the smoke, the whole corridor was filled with it. Everyone was murmuring among themselves thinking that it was a fire at the opposite classroom and why the fire alarm hadn't gone off. They were all waiting for the bell to go off to get out of class. Finally the teacher noticed it and went out to see what the commotion was all about. Apparently one of the students had 'accidentally' pressed the lever of the fire extinguisher. Our teacher scolded the boy who did it and we could see tears welling up in his eyes. All of us were giggling to ourselves trying to cover the fact that we were laughing. Finally the fog cleared up and everyone resumed their studies as per normal. However from the atmosphere of the class i knew that that incident would stick in our mind for a long time. In the end the question lingured in my mind why would someone on the fire extinguisher