Is Rav Nosson considered part of the Chareidi world or the YU world? YU has their own liberalized shittos, as classes teaching women Gemorah. (Which has no source prior to Rav Yosef B. Soloveitchek.)
I have no idea what the halachic source is – I am wondering if maybe its because of the voluntary nature of these classes? I went to Stern for a year and most girls did not take gemara classes – only those that really wanted to, did.
If you teach girls gemara in elementary or high school, that could be tiflus because you are forcing it on some girls who have no interest.