However, if my child did show a pattern of getting burner phones from friends I've got bigger problems than a smartphone. Withdrawing them from school would be a legitimate option if needed, although it would take a while to get there and would require a pretty long history of stuff like that.

I'm not so concerned about the social stuff. Smartphones are already messing up kids' social skills and introducing more social anxiety. I know it's possible to mess up your kids in other ways, but I don't think holding the line against a smartphone for longer than average will do worse than giving them one too early.