Hana to Otome ni Shukufuku wo (2009)

The protagonist Tsukioka Akira is a normal guy living his usual life, except for one thing. His twin sister is sickly and is in the hospital and can’t go to school. Problem is that she’s been sick for too long and she may have to repeat the year, or worse drop out of school. So their parents being the most clever people in the world, decided to send him, the splitting image of his sister, over to the all-girls prestigious school to go undercover as his sister, and live her life for a while so that when she comes back, she can take his place without dropping out of school or repeating. Of course it’s a stupid idea and he resisted, but him, being someone who is easily swept away, gave in. So thereon starts his incredible life in an all girls school.