About cell phone jammer?

Does anyone know about cell phone signal jammer, is it illegal to buy one? Does anyone know anything about a mobile phone jammer? I’d like to get one for my classroom. Kids are nor supposed to use cell phones in the building (landline phones are in every room in case of emergency). The kids still sneak phones in and try calling or texting on the sly. Has anybody worked with these? Am i doing the right thing then?