Firstly, I want to reassure you that your DD hasn’t done anything criminal (by sending photos of herself) and that she would not be removed from your care on the basis of that.
The fact that Children’s Services have contacted the school about indecent images indicates that they have received a report from someone about images of your DD or possibly (I would guess) multiple children at your DDs school - which would explain them speaking to the school before you.
In all honesty, without further concerns, it’s highly unlikely CS are remotely interested in your DD - they don’t get involved with every child that sends images of themselves, imagine the workload! So I would imagine this is tangential to something else.
You say you think it would be really out of character for your DD to do something like this. You know your DD, and of course she will get up to stuff that you don’t know about, she’s a teenager. However, sending out nudes of herself at random is quite unusual behaviour and would be highly unlikely to be the first indication that anything was wrong. The (more likely) scenario is that she has been persuaded by someone in school to send photos and that these have been shared around the school. This is, unfortunately, fairly common, and kids you wouldn’t expect can get tangled up in things like this. So even if it seems out of character, it doesn’t mean it isn’t possible. At the same time, if she is denying it outright then it might be a case of mistaken (purposefully or not) identity.
One possibility, which I sincerely hope is not the case, is that she may have been groomed or exploited by adults. If that is the case, it is more likely that CS will become involved with your family, but it is highly unusual for teenagers to be taken into care, and is always a last resort. (Honestly, it’s not even on the radar at this point.)
In regards to your DD, please be kind to her (it sounds like you are). If she has done this, she will be feeling ashamed, scared, out of control. She has done something stupid, absolutely, but not something wrong. She will need her mum more than ever to help her navigate this. If she hasn’t done this then thank your lucky stars and sign up to an online course with her about internet safety, and have open conversations with her about staying safe online.
I hope this is helpful.