Read Kunoichi no Ichi Manga:
On a school trip to shinobi-themed Ninjatown, a young man named Ichi Kuno accidentally springs a mystical ninja trap which sends him back to the Age of the Ninja. Having no other path, he must pose as a girl and infiltrate Kurenai Academy, a school of kunoichi students, to retrieve a mystical scroll that can return him to the future. But when everyone around him is already far better at stealth and infiltra more.. tion than he could ever imagine being, it"s not going to be easy. He"ll need to employ his own special bag of tricks, and have a lot of luck, if he"s going to last more than 5 minutes surrounded by hot kunoichi girls, every one of whom would happily kill him in a moment if he"s discovered.