Haramichuu

Haramichuu is a middle school mentioned throughout the Overflow franchise. Quite a few significant characters are known to have attended this school.

Summer Radish Vacation
Kagura mentions to Shun that her parents, Tomaru and Moegi, were attending this school when they began their relationship.

Days series
It is known that Makoto and Otome attended this school, their homeroom teacher being Ms. Arima. Other characters known to have attended are:
 * Ai Yamagata
 * Hanon Nijou
 * Roka Kitsuregawa
 * Karen Katou
 * Kazuha Nijou
 * Futaba Nijou
 * Inori Ashikaga (possibly)
 * Ikeda
 * Reisei
 * Ii