Rude photo being shown at school

[Namestheachanging123]

Ok can't seem to sleep so seeking some advice.
Picked up my dd10 from school yesterday and she complained to me about a boy in her class. I probed her some more but she didn't explain.
Once home she mentioned him again and I asked again what he'd done, expecting her to say the usual minor irritations that she tells me.
But no, she told me that this boy had been showing her class mates a picture of her that he secretly took ( on his phone ) and has made it look like - to quote her exact words 'like shes having sex with herself'
Obviously my jaw hit the floor and I blurted out that that was not on and I would need to speak to the teacher about it.
She was immediately defensive about it , didn't want me to speak to the teacher, wished she hadn't told me about it etc.run of to her bedroom.
I left her there to cool down;we were literally about 10mins from leaving the house to go and celebrate my birthday with family and came home quite late so I have yet to go into it further with her.
My question is how do I proceed next?
I will definitely be questioning her more on it in the morning but should I go directly to the teacher about it or speak to the mum first?
I do know her to talk to and know for sure she won't go easy on him about it.

I’d speak to the school and his mum if you know her also I’d seriously consider phoning the police of the picture has been edited in to something sexual.

Schools can brush stuff under the carpet and I’d want her to know.

That boy needs the biggest shock to his system over what he has done.

Schools can brush stuff under the carpet

Schools are absolutely not allowed to brush things like this under the carpet. It you have become aware of and reported something similar and you know it hasn't been taken seriously (bearing in mind you won't be given feedback) then you need, for the sake of the children involved to re-report it, to the Police, NSPCC or Social Services yourself.

I’d want her to know
The child's parents will find out soon enough, but will be told by someone trained in how to handle such things so the children involved are protected and, if a crime has been committed, any evidence is not lost.

That boy needs the biggest shock to his system over what he has done.
Possibly. Or he might be in need of protection himself. There may be other children involved, as victims or otherwise. We don't know, which is why the correct route is via the Safeguarding Officer.