This is confusing. The whole inyan of niddah is brought down in Chumash very clearly. Are BY girls learning Chumash with Rashi and other meforshim and somehow not noticing? Are the teachers not teaching, just saying, “Don’t ask about that?”
And I agree with those who say:
A) You have to know a lot if you’re going to go out into the world to work. Offices can be very iffy places, with yichud issues, negiah (even by the innocent but ignorant) and so forth. And not just for girls. Guys can have problems too.
B) If a girl doesn’t get some serious hashkafahs on this subject she is going to be open to influence by secular culture, and it’s almost impossible to avoid, particularly if she’s working.
We can’t insist that our girls go out to work to support their husbands and leave them without the knowledge to protect themselves.