12-Sai. Chiccha na Mune no Tokimeki (Age 12. A Little Heart-Pounding)
The story follows Hanabi, a sixth-grade girl who is neither an adult nor a child. She deals with issues such as accidentally witnessing her homeroom teacher kissing, and worrying about experiencing physiological changes before her friends. This is the depiction of a 12-year-old girl's innocent worries and first love.
[Source: Anime News Network; emended]