In the early episodes of Tokyo Ravens, Natsume is seen wearing a boy's uniform. It is said that this is due to some customs in their family. As quoted from Wikipedia here:
As a family tradition, she has to present herself as male in front of other onmyouji families, a tradition that was created to hide the true identity of Yakou's reincarnation.
But why didn't anyone notice that she is actually a girl? Everyone except Touji and Harutora who already knew her, assume that she's a boy.