Advising girls to diet

[Antoni]

Today my 15yr old dd came home and told me that the headmistress advised her to go on a diet. What should I do about this. My dd is a size 10

Well, I doubt the head walks round accosting pupils at random. Did your DD explain any more of what led up to this?

Could this possibly be a 'sensitive-teen' embellishment of what was actually said OP?
If not... shocker!

I cannot believe that was said!

I would go in and ask for the school's version of events.

If their story suggests this did happen, say how unacceptable it is.

If they suggest nothing like this occurred, say how you thought this might be the case, because otherwise it would be so unacceptable.

In other words, make the case regardless of what the school says happened.

Was there more context than just the head walking up to your DD and telling her to go on a diet...
Was your DD eating a chocolate bar or something she shouldn't have been due to school rules?
The head at DDs school once told her jokingly off for rolling her skirt up by saying she was looking rather chubby and perhaps if she unrolled her school skirt which was creating a bulge around her hips, then she would return to her normal healthy weight.