I went to a school with a relatively strict no-phone-in-class rule and it was the most nonsensical rule of that school. Students simply carried second phones or said they forgot it at home that day. Despite almost all teachers enforcing that rule, there were still at least 5 people in every class who had (and used) their phone. Afaik it’s still enforced nowadays, but there was and still is no apparent effect on grades or quality of the classes.