Missile to Plankton

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.

The manga follows with their stories and the stories of the other missile club members.

(Source: MangaHelpers)