As a secondary teacher I am probably more aware than many parents of what kids are passing back and forth. I don't just mean stuff with sexual content/sexting- though this is a big issue, obv - I'm also taking about horribly violent videos, bullying messages on snapchat etc, being groomed for county lines - all sorts of stuff that harms the children.
If I could offer one piece of advice to parents of kids this age, it's this: You probably pay the bill for this phone. So it's YOUR phone, you just let your child use it. That means you can check it At Any Time - if the child refuses, there's possibly concerning stuff on there so I'd suggest a stop on the account and a brick phone for a few weeks. This can be phased out as the child grows up - I stopped checking my dd's phone when she was towards the end of y10 I think. But it is vital in those end of primary/ start of secondary years.
Please, for your child's sake, check their phone.