Joseph,
It is not an issur for a woman to learn and it is not even an issur d’rabbanan to teach a woman. Rather Chazal said (according to R’ Eliezer) that a man should not teach his daughter (i.e. as a child) Torah because most women do not have the right type of mind for learning Torah sh’bal peh. However if she begins to learn on her own and he sees that she is among the capable minority, then she can be taught and she receives schar for her learning. This is meduyak from the Rambam (Talmud Torah 1:13), Tur (Yoreh Deah 246), and Shulchan Aruch (Yoreh Deah 246:6), and is mefurash in the Perisha on that Tur. This is how there were individual scholarly women throughout the generations.
It is also meduyak in the Gemara in Nedarim (can’t look up the cite right now) that there were girls schools in Mishnaic times (although they may have just held like Ben Azai).