Why didn't anyone in the school notice that Natsume is actually a female?

Question

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.

Answer 1

There are several reasons why no one noticed that Natsume was a girl:

1. She was dressing as a boy, therefore no one should suspect her.
2. On the novel, an onmyouji can identify whether you're a girl or boy based on your aura. Boys naturally have yang aura and girls have yin aura.
   
   Normally, when a practitioner has a shikigami, the shikigami's aura should be the opposite so that their compatibility will be in harmony.
   
   However, Natsume's prideful shikigami which is Hokuto have a yin aura, therefore it made conflict in their auras, thus negating the yin aura and making it a yang aura. In result, anyone would assume that Natsume was indeed a boy.
3. It is rumored that Natsume was Yakou's reincarnation, since she was the next heir of the Tsuchimikado family, they preconceived that she is a boy.

Answer 2

After finished watching the anime's first season (hoping for a 2nd one), I have now an answer to my own question.

Just to clarify things, (SPOILER on hidden text)

• Natsume being a girl was revealed on Tokyo Ravens episode 17 and her being a boy was explained then from episode 18, 19 through episode 20.

• Natsume being a girl was exposed when Shaver was out of control and started attacking them after a spirit disaster was unleashed.

  Shaver tore Natsume's clothes apart which revealed the truth.

• Jin Ohtomo Sensei and the principal of the onmyouji academy as well as the Yakou fanatics and the twin horned syndicate never noticed that Natsume was a girl, until it was revealed to them. According to Jin Ohtomo, a B-Grade Spell was used and, although noticeable, if carefully examined via one's aura, it was never revealed. This can be linked to catzilla's answer:

  However, Natsume's prideful shikigami which is Hokuto have a yin aura, therefore it made conflict in their auras, thus negating the yin aura and making it a yang aura. In result, anyone would assume that Natsume was indeed a boy.

• On episode 20, it was revealed that Natsume's father told her about the family tradition when she was in junior high. After that, the times that Harutora came to hang out drastically reduced.

  That was when Natsume decided to make the familiar Hokuto to continue hanging out with Harutora. although this was a secret even to Harutora.

• The cross-dressing was endorsed by Yasuzumi Tsuchimikado to hide the identity of the reincarnation of Yakou (Polaris King)

  The real reincarnation was not Natsume but Harutora Tsuchimikado. It was also to conceal Natsume being a fake as she was a girl. From the moment they were born they switched Natsume and Harutora was set on the branch family. Harutora who has tremendous aura and energy, was put under a curse to hide this and also concealed of his abilities one of is being a spirit seer. The curse was confirmed by Kakugyouki the one-hand ogre.