Cell phones are extremely distractive in a class because the majority of the students would text, each of play games on their phone.
In a recent survey, taken by a teacher, students who bring phones to school were asked their marks in the end of term exam. The average score is around 52%.
I believe cell phones are distractions for students. If cell phones were allowed in school, student would spend more time with their phones, become obsessed with it and eventually neglect studies.