Pretty much secondary school in the UK (12+). I'm guessing middle schools / high schools in the US are the same? Yes, literally every kid in secondary school has a cell phone. Kids have whatsapp groups and communicate all the time with their friend circles.

According to an Ofcom survey in 2023, 9 in 10 kids aged 11 have a smartphone in the UK.

1: https://commonslibrary.parliament.uk/research-briefings/cdp-...