"If you have a phone in class then _you_ leave the class."
Why is the school treating these children like clients? They aren't. If they can conform to the rules, they can come, if they can't, they don't. You don't have a natural right to be in the classroom.
I’m pretty sure that, as a matter of law, school attendance is compulsory in most western countries. Whether or not students are “clients” is immaterial; the school has an obligation to teach them, and the students are compelled to attend.
I think the one place that I used my phone the most in high school was in suspension!
(More broadly: I don't think it's remotely practical to remove students from classrooms like this. To a first approximation, every single student in school has a smartphone.)
Why is the school treating these children like clients? They aren't. If they can conform to the rules, they can come, if they can't, they don't. You don't have a natural right to be in the classroom.