Haven't read the whole thread, but enough.
Some of the posts are demonstrating 'MN at its most ridiculous'.
I am a former long-term breastfeeder (one child, four years three months feeding), and volunteer breastfeeding counsellor (twelve years) so I am aware of the functionality of breasts.
I am a former secondary school teacher (two schools, twenty-one years) so I have experience of the school environment and of young people.
I am a feminist. Perhaps not like the ones you meet today, but a feminist all the same.
The 'she should swing free' camp are talking total and utter crap.
Breasts don't just have a single function. They have a secondary, but important, function. That is, to attract the attention of males to promote the female's chances of breeding. No matter how loud you shout, MN, that is the plain fact.
During the few years just after puberty the instinct to make sexual contact is very strong, for young women and for men. In this period, they are in secondary school. They are still legally children but they have adult desires, often unformed, often unexpected. Sometimes the nature and extent of the sexual interest in each other is embarrassing for them and for others around them, for example, for young people who have not yet passed through puberty, or for those who, quite reasonably, just want such things to be private.
Saying this is not so, or ought not to be so, won't make it go away. It isn't 'sexualisation'. Its nature, the way it is before society gets to it.
The way forward is to teach young men and women to respect themselves and each other, whilst simultaneously providing a 'safe' environment. Schools support their safe environment by encouraging, or insisting, that pupils wear uniform. All the standard clothing worn by adults in a formal environment is included.
And that includes underwear.
The (even unspoken) rules extend to teachers, too. I've known staff be 'spoken to' about being braless or wearing inappropriate clothing.
OP, insist your dd wears a bra for school. At home, she can do as she likes. But school is a place of work, and the formalities protect the boundaries between pupils and between pupils and staff.
[And the poster who said she has married a man who doesn't look at women's breasts - one way or another, you are going to be disappointed.]