Terry Pratchett 
Equal Rites 
(Discworld Novel 3)

”They say that a little knowledge is a dangerous thing, but it is not one half so bad as a lot of ignorance.”
Everybody knows there”s no such thing as a female wizard. So when the wizard Drum Billet accidentally passes on his staff of power to an eighth daughter of an eighth son, a girl called Eskarina (Esk, for short), the misogynistic world of wizardry wants nothing to do with her.
Thankfully Granny Weatherwax, the Discworld”s most famous witch, has plenty of experience ignoring the status quo. With Granny”s help, Esk sneaks her way into the magical Unseen University and befriends apprentice wizard Simon.
But power is unpredictable, and these bright young students soon find themselves in a whole new dimension of trouble. Let the battle of the sexes begin . . .
” If you”ve never read a Discworld novel, what”s the matter with you?” Guardian
” Pratchett uses his other world to hold up a distorting mirror to our own” The Times
Equal Rites is the first book in the Witches series, but you can read the Discworld novels in any order.