Allow phones in school. This partially mitigates the problem by allowing neglected children to call for help.
Don’t allow phones in school. This does not at all mitigate the problem; neglected children remain helpless.
There is no third option of solving the problem of children being neglected in schools. That would require people who don’t care to magically start caring, which obviously isn’t going to happen.
Therefore, the greatest harm reduction is achieved through option 1.