Sweet Blue Flowers(2004)

Fumi and Akira were best friends when they were little, with Akira always looking after the crybaby Fumi, but that all ended when Fumi's family moved away. Several years later, Fumi's family returned, and she and Akira happened to bump into each other on their way to school. They became friends again, quickly slipping back into old patterns. Shortly after, Fumi began dating a cool, attractive upperclassman who, coincidentally enough, had ties to Akira's current school, the prestigious Fujigaya Girls' Academy.

(Source: Lililicious; updated from an early misreading of the school name Fujigaya, called Fujigatani in initial translations of the first chapters)

Note: Includes 10 extra chapters.