Kaichou no Kirifuda

There are three high schools: co-ed Narayama High, all-boy Kashimori High, and all-girl Sakuragawa. Due to decreasing enrollment and financial difficulties, the three schools must be merged. However, those in charge of the reorganization could not agree on which schools to close and which to keep open. They finally decide to let the students themselves determine which school will survive. Now all the students have the chance to protect their schools, but which one will be the last one standing? The student council of Narayama High, at least, don't want to give up on their school without a fight!

(Source: MangaHelpers)