All the Bad Things.

Hinako, the student council president, secretly pines for the council's advisor Yano-san. When Chibana, an underclassman on the council, finds out about Hinako's secret, he forces her into a "mutual agreement." The straight-laced council president and the scheming kouhai, all alone in the student council office after school with their shared secrets. (Source: Kodansha, translated)