Maybe it's a translation issue, but it seems in this article that the effect on school work (i.e. tests, quizzes, reports, homework assignments, etc.) was not studied, as I think the title of the article implies it was. Rather, they studied the phone ban's effect on school culture and bullying.

To be clear, I believe a phone ban in schools would have a positive effect on academics, and that that would show up if it were studied. It's just that it doesn't seem like that's what they did in this case. Thus, arguing about whether phones affect attention span, memory, even reading time, is not relevant in the context of this article.