The Sealed Auditorium

The sealed auditorium is said to be a special building in Kamome Gakuen Academy. It's only open once a year for the School's Fall Festival, where middle school and high school first years use it to perform a play of their choosing.

66th Festival
During the 66th festival, Hanako and Tsukasa were first-years in middle school, which means their class performed a play in the unopened auditorium. Neither of them had acting roles, and only Tsukasa helped with the preparations.

113th Festival
In the current day, Kamome Gakuen begins its 113th festival, where the play "The Wizard of Oz" will be performed by Class 1-A.

Trivia

 * The School Festival has been going on since at least 1902, assuming that the 66th Festival takes place in 1968. Using this estimate, the 113th Festival takes place in the year 2015. Assuming that the Sealed Auditorium building was already existing by the 1st Festival, it can be presumed that it was only used for a total of 113 times.