Recorder and Randsell

Recorder to Randoseru (リコーダーとランドセル, lit. Recorder and Backpack) is a yonkoma manga series by Meme Higashiya. It will be adapted into an anime in 2012.

Story
The manga revolves around Atsushi Miyagawa and his sister Atsumi Miyagawa. Atsushi looks like an adult at 180 centimeters (about 71 inches) tall, but he is actually an elementary school boy. Atsumi looks like a little girl at 137 centimeters (54 inches) tall, but she is actually in high school.