Sanshu Middle School is the school that the main characters of Yuki Yuna is a Hero attend. It is located in the Kagawa Prefecture of Shikoku.
During season one, the Hero Club members would end up on the roof after they completed battles (there were exceptions to this, however).
Sentinels are forbidden from entering the premises of this school, let alone attending it, due to the Taisha not wanting them to make contact with the heroes. Kagajou Suzume, however, was able to make an exception and meet them briefly.