Earlier quoted context omitted.
> a child complaining about anonymous abusive messages is ignored by the school. School's not responsible, because the child's phone is set up improperly. Imagine if the police requested, but technically didn't mandate, that parents install location tracking apps on their children's phones, and that they then ignored missing persons reports for anyone who didn't do so.
Quoted post unavailable.
In any case, I don't think a technical countermeasure that addresses a subset of cases erases the concern. You're presupposing harassment happens by SMS, which maybe was true 10 years ago.
How would your solution work with students' frequent use of Discord?