Have we got a link to said trousers?
Are they tight round her bottom and thighs?
As someone else has said, surely if she is tall and thin, said trousers would be automatically a bit baggy as you often have to go up a size to get the leg length right.
Are they "fashion" trousers rather than school trousers?
Was the word used by the teacher "sexy" right from the start? What I mean is, did the teacher use other phrases first, but then expanded or changed term to "sexy" if/when children said they didn't understand what she meant initially?
As mp says, I would think "sexy" was an inappropriate term to use to descirbe children's clothing to an 11y, but not horrnedously so, just ill judged and not right for the audience.
The jiggling - again, wouldn't be too happy I guess, but not overly upset and personally would be something I would be complaining over. I am assuming a teacher just held a bit of fabric and jiggled it a bit. It is definitely not inappropriate touching, unless skin contact took place, or it was holding material near thighs and bottom - then more suspect.
However, should add that when I see what some children, especially girls, turn up to school wearing, and hinking it is appropriate school wear - well, some of it would look too revealing for a night round town, let alone in a classroom. I can see why schools need rules.
Oh - and at 11y I am assuming it is secondary school and, afaik, secondary schools in the state secotr can impose uniforms.