Kunoichi no Ichi! no Ni is a Manga/Manhwa/Manhua in english language, SUGGESTIVE series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

A high school student Ichi who visited a ninja residence on a school excursion was caught in a strange structure during the tour and time slipped to the ninja world. Along with his friends, Ichi who got the “Ninpocho”, a key to return to the present age, returns safely to the present age. However, suddenly an awakening appears in front of Ichi who lives a peaceful life ...!?