I let DD have it at 11 just before she started secondary school. But we set a few ground rules.
Firstly I said that I needed to be able to look at her phone and messages at any time, and this was one of the conditions of having a smartphone rather than something that would only make calls and text (and I said any time there was a problem with the content I would be altering phone arrangements immediately). I did check her messages, not every day, but often enough to monitor what was going on in the groups she was in. She's fifteen now and the deal is still that she cannot refuse to show me anything on her phone or we will instantly be downgrading. If she changed her passcode so I couldn't get in that would be a dealbreaker.
I said she couldn't join any chat where there was someone she didn't know in real life as part of the chat. She was added several times to big group chats with children from other schools and the deal was she should show me what was going on and then leave the group. She has stuck to this.
I said she couldn't join any very large chats bar the one for her old primary which was a class of about 28 and most of them were on it. There was a whole year chat at her new school and I said that was too big and she could not join it, and she didn't. Subsequently it turned out that there had been some rather horrible behaviour on this group which ended up having to be dealt with by the school so I think she ended up glad she had not joined. I think big group chats have a horrible dynamic in teenagers and nobody needs that in their lives!
I said she needed to tell me if anything at all happened that she was uncertain about or found scary/mean and she's stuck to this too. There were a couple of incidents that I had to speak to other parents about (luckily the parents were friends of mine so it wasn't awkward).
Mostly it has been fine, BUT I have a very compliant, kind, well-behaved child who is keen on rules and likes everything to be calm and ordered. I am not sure I would have let her have it if I couldn't have felt that I absolutely trusted her to behave sensibly and let me know if anything bad went on.
DD found the familiarity of the old school group chat comforting at first, but as she found her feet at secondary she realised she didn't need it any more and ended up leaving it around halfway through year 7 after some arguments. She still kept in touch with her real friends from her old school, but didn't want to be part of the old chat any more as she had found new friends and a new identity at secondary.
Don't know if this is helpful, but that is what we did and it worked out fine.
Also, you may find that your child's friendship group changes a lot now so not being part of a group from her old school may matter a lot less to her in a matter of weeks.
And yes, they do use it to discuss homework etc (lots of sending each other pics of worksheets if forgotten at school or photos of pages of a textbook) so I would probably give it a try in your shoes and monitor what is going on so you can see it's all harmless.