ONIMAI: I'm Now Your Sister!

Mahiro Oyama was just a normal erotic-game loving dude…until he woke up one morning as a woman! Turns out his mad-scientist little sister, Mihari, tried out one of her new experiments on him…with a disastrous outcome, as far as Mahiro’s concerned! But Mihari is as determined to study him as he is determined to go back to his shut-in, game-playing life, and one thing’s for sure…life is going to get a whole lot weirder from here on out! (Source: Kodansha USA)

Kyou no Go no Ni (2008)

Following the line of the precedent OVAs (of the same name) This anime goes around the normal lifes of some elementary school (5th grade, 2nd class room) kids and their adventures, while turning to teens. Their games, stupid jokes, and the misunderstadings around their premature attraction to the opposite sex. Ryota Satou is the main character here, but his friends are almost as important as him since the school wouldn't be the same without them, specialy the girls who the majority of time appear to be mature by their ages but in the ends they are as childish and even more "dangerous" than the boys. (Source: Anime News Network)

My Wife is the Student Council President

The story begins with Izumi Hayato running to be student body president. But when a beautiful girl swings in promising the liberalization of love while flinging condoms into the audience, he ends up losing to her and becoming the vice president. At the student council meeting, the newly-elected president invites herself over to Izumi's house, where she promptly announces she is to become Izumi's wife thanks to an agreement – facilitated by alcohol – made between their parents when they were only 3.