To me a phone is inherently private - you don't let your kid have a phone if you can't respect that privacy and don't consider them able to handle the responsibility. Looking at it is like reading their diary or listening in on their conversations with their friends.

Though how realistic it is to try to keep a high school age kid from having a phone these days, I have no idea - seems as if it would be easy for a kid with a bit of money to get a burner phone and a pay as you go account and hide it from their parents. My kids (now in their twenties, so the tech and the societal expectations around it have changed a bit) did have smartphones in high school, and non-smart cellphones in late middle school.