At around 1:00 a.m. on February 21, nine men wearing stockings and white gloves appeared in front of a janitor (38 years old at the time) who was on his night shift and threatened, "We will not steal or set fire. If you do nothing, we will not cause any harm." After that, they tied him to a chair with adhesive tape and plastic straps, locked him in the bathroom, and disappeared somewhere. Three hours later, when the janitor sensed that there was no sign of presence, he escaped from the bathroom on his own and came out to find that there were 447 desks and 9 chairs in the center of the playground, measuring 20 meters wide and 30 meters long.

The desks were arranged in the shape of the number 9 and the chairs arranged in the shape of a period next to the desks. Japanese news and the public spent days speculating what it meant. Theories from the occult to a protest of the Japanese government were seriously discussed on national TV.

In the end, the people involved were caught (all senior students of the school, the leader with a history of causing trouble) and the leader of the group admitted that he simply liked the number nine.

https://en.namu.wiki/w/세타가야 '9'자 사건