In the past, rainbow flags were used as a sign to students that you as a teacher were available for anyone to come talk if they needed support, and that your classroom was a safe space. This was especially important for some of the most stigmatized and marginalized groups in the US.

So there is a bit of history behind it.

This may only be anecdotal, and I can't quote statistics on how effective or important pride flags are to that type of support now, but a close friend of mine who came out in high school first came out to a trusted teacher. They ONLY did this because the pride flag to them meant someone who might understand. It was a year or so later when this person came out to our friend group, and only because of the guidance of that individual who help them through some serious shit, and got them the support infrastructure they needed.

There is literally nothing wrong with the pride flag (or progress flag). There IS a problem with the confederate flag.

I would much rather my son or daughter learn to be accepting of their neighbors regardless of their sexuality, than to learn whatever kind of bullshit a pro-slavery person has to tell them about their "history".

So, if there is a school club they can't let people in the school know about it with posters, even though that's the only people they are trying to attract?

Dude, I mean high school clubs advertising to other high school students, like this one: https://www.coolidgeshs.org/ourpages/auto/2023/5/3/45488112/image%20_2_.jpg