Welcome to Wakaba-soh (2007)

Blurb:
In the throes of his first love, Kentarou Sawai can’t even count on beginner’s luck to get him closer to Karen Toguin, the girl of his dreams. When he spontaneously enrolls in Karen’s high school just to be closer to her, his hopes are crushed when it turns out she’s on a leave of absence! Now poor, luckless Kentarou is stuck in a dorm full of girls who aren’t Karen and insist on pushing Kentarou to the brink of insanity! Can Kentarou hold on to his love for Karen, or will his new living experience make him swear off all women for good?!

Source: www.anisearch.com/manga/4227