The school is in my area.
I can't actually see anywhere in the letter where it says the skirts were 'distracting' just that they didn't meet uniform requirements?
Their webpage says they have guidelines on uniform, though I can't see them so they may be in their planners or somewhere else.
These girls are 'normal' teenagers. They aren't massively overweight or anything. I don't see how all the other girls are finding skirts that meet requirements and not these two.
My guess is that there is something in the guidelines (e.g. not figure hugging, and no more than x inches above the knee) that the girls aren't meeting.
Personally I think they look fine, but I suspect there is more to this than the DM has reported.