Cellphone jammers do exist, but legality is probably an issue. Especially so now that the government is using cellphones as part of the Emergency Alert System.
You are not allowed to use a cell phone jammer in school. This is a pity, because despite official school rules banning them, the rules are rarely enforced. Teachers feel foolish continuing lessons while nearly every student’s attention is riveted to that plastic toy in their hands. You might as well recite the phone book as much as they are listening. Then they want to know why they fail classes. I also just love when students take unauthorized pictures of their teachers in class to embarrass them, record what they are saying and edit it to get their teacher in trouble, and take pictures of their exams to text them to friends in the classroom.