Missile to Plancktone

Tatsuko Sachi was a stupid girl who remade herself as a "genius" at a prestigious school in order to be near the boy she liked, Oumi Shouta. When Shouta decided to make a club to build missiles, she then agreed to be vice president. However, Shouta slacks off and leaves most of the work to Sachi. This becomes worse when a child-like girl named Onda Midori shows up, and Shouta spends all his time doting on her. Not even having super powers makes this situation easier to handle.

(Source: MangaHelpers)