To cut off students from the outside world / their parents, it has to be ensured that they do not smuggle communication devices can be smuggled in, while teachers freely show each other social media clips on their phones (happened during my school time).
Smartphones are neutral pocket-sized computers. They can be used for productive puporses or otherwise. If smartphones are so distracting, teachers shouldn't have them either. Teachers use the same social media they say distracts students.
Another reason is to prevent students from contacting their parents when teachers engage in bullying.
Date: 2030-2035.
Evidence: None, but it is plausible. "Public" schools (meaning government-backed) will have no trouble funding it. I wouldn't be surprised.