I'm sitting here from the perspective of having 2 adult children -- both graduating early, and my youngest who enters high school next year. They all got phones starting with 9th grade (14 at the start of the school year). Generally speaking they charged the phones downstairs.
I literally addressed the problems of missing out for high schoolers. The social event they'll miss on its own may not be a big deal. But the missed opportunities to make and deepen friendships is a big deal.
As an adult, you have your friend/peer group already. You and your friends are far less likely to do things spontaneously and far more likely to be courteous and thoughtful to invite someone along. A high school teen has middle school friends and is probably discovering their peer group. A huge portion of that discovery becomes unavailable without a phone.
Say they just finished up with band and everyone chats online, "hey meet up at the pizza place." Your kid will be left out. That's the firm reality. They won't know about it. It's the opportunity cost of meeting and learning other kids which will be missed, not the riveting conversations themselves.
> They are not required, despite what you may have been told.
Required? no. Central to the social fabric that binds them, unfortunately yes.
I'm fairly confident in my methods as I'm 2 for 2 graduating conscientious adults from college.