Since you mention violence against students, let me put it this way. Phones make it easier for the offender to plan an offense in schools. We know how school bullies can be.

Also taking expensive things to school will always result in increase in thefts and other such issues. Like Smith mentioned, it will take a big toll on their mental health too. Phones being allowed in schools can also result in digital harassment. Making unsolicited videos of others for likes on tiktok, planting a phone in private areas of the school like rest rooms and a lot more. The cons of allowing phones outweigh the pros.