I think this is a perennial condition for most theatre teachers.These Shining Lives, Radium Girls, Triangle are great for drama. Classics are a god bet as well - Greek comedies can be very flexible (I've adapted both the Birds and the Frogs with large female casts)
I've also written a lot of full-length, large-cast female shows. Game of Tiaras (think Game of Thrones but with Disney Princesses as the leaders of the houses), and Empowered (basically Glengarry Glen Ross with girl scouts) are very popular.
You could do almost any fairy tale/kids show situation and make animal/fantastical parts girls. Fabulous Fable Factory? I adore Law and Order - Fairy Tale Unit. I have also had this problem and have even gone searching for boys in my English classes!
My favorite show, which I have done more that once, is Up the Down Staircase. You could make all of the teachers girls, including the principal. You would need one male teacher (Paul) and a Joe Ferrone - although I suppose Joe could also be female... now that I think about it! It's a large cast and has a great message.