I'm out of school for 3 years. For most of the time smartphones existed, but were forbidden in class. It were the teachers, who allowed it and even encouraged it to be used in class. When the students are already staring at a screen during class, they continue doing so after. When they do not, they don't (at least initially). In my final years, it was obvious who thought about class and who was completely mind-absent, by looking who looked at an school-supplied iPad. > 90% of these people were just playing games during the whole lesson. This was known by everyone in the classroom and the teachers just ignored them.