Tsuma, Shougakusei ni Naru. Hatsukoi Aite no Kimi wa Dare?

Takeru, a fourth-grader in elementary school, starts to take notice of his classmate Marika, who has recently started acting like a more mature girl. It is the beginning of the first love of an innocent boy!
(Source: Shukan Manga Times, translated)
Note: Includes 1 extra chapter.