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 Fall Festival, where middle school and high school first years use it to perform a play of their choosing. It is home to a clock that had stopped working long ago, which is revealed to be the clock that the Three Clock Keepers watch over in their rumor.

The Clock
The Sealed Auditorium is home to a large and mysterious clock that has stopped working at some point before Amane and Tsukasa Yugi were enrolled in Kamome Gakuen. At some point, Amane had tried to fix the clock, but was unsuccessful, and the clock remains unfixed to this day.

According to an apparition of Yugi Tsukasa that appeared alongside a time-displaced apparition of the 66th Festival, there is a rumor that if the Clock in the Sealed Auditorium is fixed, it can allow the one who fixed it the ability to travel to any point in the past or future. Tsukasa used the rumor to tempt Yashiro Nene into fixing the clock so that she can find out more about Hanako's past.

In Chapter 103, Hyuuga Natsuhiko implies that the Big Clock is the location of School Mystery Number 1's Boundary, and in the following chapter, he comes to the Big Clock, spills his blood, and lets it coat and permeate the entire clock. All the while Natsuhiko claims that his blood has the power to instantly kill off the Clock Keepers should they make contact with it.

In Chapter 105, Natsuhiko's blood destroys the Clock, stopping time as per the rumor, and leading to Yashiro Nene, who had investigated what happened to the Clock alongside Hanako, to be captured by Yugi Tsukasa (after he kissed her lips in front of Hanako). It is currently unknown whether Kako and Mirai were actually affected by the event, however Tsukasa implies that they are still around, and that in order for the Academy's time to be restored, they have to be defeated and their Yorishiro removed.

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.