Various Opinions from the Imams of the 4 schools of thought:
Imam Abu Hanifah: If a lady leads the salāh of a purely female congregation, then salāh will be correct. It is however makrooh tahrimi for women to form their own congregation.
(Hidayah vl.1,pg.305; Bada'i-us Sanai vl.1,pg.157)
Imam Malik: A lady can in no circumstance be the Imam even if the congregation be entirely female. The salāh of even a lady behind a female imām is invalid.
(Bulghatus Salik vl.1, pg.146; Ashalul Madarik, vl.1,pg.241)
Imam Shafi`i: A lady can be the imam of a purely female congregation. In fact it is mustahab for them to form their own congregation.
(Al Mughni vl.12,pg.199; Bada-i vl.1,pg. 157)
Imam Ahmad: The salāh of a lady behind a lady imām is permissible. There is however difference of opinion regarding women forming their own congregation (behind a female imam).
(Al Mughni l.12, pg.199)