So Cute It Hurts!! Vol. 1 (manga)

The Kobayashi twins, Megumu and Mitsuru, were named after historical figures, but only Megumu has grown up with a taste for history. So when Mitsuru is in danger of losing his weekends to extra history classes, he convinces his sister to swap clothes with him and ace his tests! After all, how hard can it be for them to play each other? But Megumu can't rely on just her book smarts in Mitsuru's all-boys, delinquents' paradise of a high school. And Mitsuru finds life as a high school girl to be much more complicated than he expected!