True Love Plus Extra(2000)

Renai Plus α

During a spring school vacation school teacher, Natsukawa, goes out searching for his delinquent students and ends up finding one in Koga he wished he hadn't hunted for.

Series also contains funny and interesting oneshots. A third book in the works provides a new story as well as 3 chapters of Renai Plus Alpha.

(from hochuuami.com)

Volumes which feature the main couple go chronologically as:

Renai Plus Alpha v01
Renai Plus Alpha v02
Yappari Kimi wo Suki ni Naru
Love Masshigura!

Prequel: Ren'ai Liberty!