My dd once had the same for the colour of her bra
It was white and not flesh coloured so it was visible under her shirt
I was hauled in and patronised
I went mental-nobody should have been looking at her underwear and it certainly didn't impact on her education
It ended with them threatening social services and me threatening I was going to the newspapers,sm and my mp
Our cards where marked that day (being a working class,single parent didn't help-that teacher hated the working class and/or single parents)
Ds went to another school who insisted on trousers,shirt,tie,blazer,white socks and black shoes (no handbags allowed for the girls and no brand named bags for both sexes)
All good-but in hot weather they where not allowed to remove the blazer and in winter they where not allowed to wear 'outside coats'
It's fucking barmy-report bullying and they don't want to know but dare to wear a white bra and they are all over you
(ds was once attacked by bullies on school grounds and they just shrugged-i had to get the police involved before they pulled their finger out-same kid loosened his tie a week later and he got a dentition)
Normally I backed the school up and tried to work with them but some rules where just pointless