I am sceptical of dd2, not because I think being bi is a myth, but because at the moment, at her age, and with the current situation, I think she is jumping on a bandwagon.
I'm sure you don't mean any harm but "jumping on a bandwagon" is very dismissive, about bisexuality as such and also about girls trying out different labels to explore the different aspects of who they are and what they really want. And of your younger DD.
all the girls at the secondary school ... It is currently, amongst that group, cool to identify as anything other than straight.
And that group is all the girls at the secondary school? Well, if you say so, maybe it's a small school with a uniform demographic and nearly every girl in the whole school has the same idea of cool. But you also said yourself, plenty of "that group" casually identify as straight at least some of time so heterosexuality can't be that uncool.
The fact that these different labels are even openly accessible for (some) youngsters to figure out what they want is an achievement. Whether the results please their mothers or not.
Back in his 80s activist days back when people talked about Boy George, bandwagons and confusion every time bisexuality was mentioned, so things have clearly changed a lot, no Boy George DH owned a T shirt with slogan "I'm bisexual, you're confused".