Now you have a clearer picture I'd definitely raise it with the school and follow up your phone call with an email asking what's going on rather than hurling accusations.
The only thing that surprises me is the presence of the other teacher. That doesn't mean DD is lying, it might be that you got the wrong end of the stick.
It doesn't surprise me at all that this is happening and I am having a very hard time trying to think of an innocent explanation.
When I was about 12 the school caretaker wandered in on us changing after swimming two or three times. The layout was separate male and female changing rooms which accessed a single one for the pool which would only ever be used for a single sex group.
He'd enter from the boys' side and make the excuse that he needed to do something urgent to the pool and was coming through despite our upset. He was never apologetic about it. He was very bold. Looking back he probably got even more of a dirty little thrill ticking off naked 12 year olds.
Our teacher happened to be out of the changing area - probably getting ready for her next lesson - the first times he did it and I suspect he knew that.
We never thought to tell her or anyone else. We just thought adults could do that.
. Unluckily for him she popped back in unexpectedly. She went mad. He never did it again. I don't doubt that she complained to the head about it but he didn't lose his job.
BTW I suspect she was gay. But she didn't show any sexual interest in children. Why would she? He did though.