Among the so-called "seven school mysteries," there is traditionally one called "the growing staircase." In the classic version, there is a staircase somewhere in the school where, if you walk it up or down at night, you will count one more step than normal. And usually it goes that the one additional step is actually someone's corpse.
It's no surprise that at K-san's middle school there was also a set of "seven school mysteries," and among them one called "the growing staircase." This story concerned a staircase near the biology room in an old school building two stories tall. Normally this staircase had 13 steps, but it was said that late at night at 2AM, if you tried counting the steps as you went up or down the staircase, you would never reach the end.
A while back there was a new student who had heard this story and decided he had to find out for himself whether or not it was true. One late night he invited a friend to sneak into the school with him. (This story is from a time when it was not yet very difficult to sneak into a school building.)
They hid themselves in a dark classroom and waited for the time in question—2AM. There were no lamps on in the school or on the grounds, and they could not turn on their flashlights out of fear of alerting the security guard. Luckily the moon was bright that night, and soft moonlight entered through the window.
It was a gentle, warm night.
When 2AM drew near, they made their way toward the staircase near the biology classroom. In the version of the seven school mysteries that circulated among these students, it was unclear whether the strange phenomenon would occur when going up the stairs or when going down them. After some discussion, they decided he would be the one to go down the stairs and his friend would be the one to go up.
At 2AM he was on the second floor. His friend was on the first first floor. They called out to each other and both took the first step at the same time.
From the top and the bottom of the staircase, they continued while nervously counting the steps. He and his friend were counting quietly, but their voices echoed in the empty staircase, so they could hear each other clearly.
When they counted the sixth step in unison, he and his friend both reached the landing in the middle of the staircase. They turned and faced each other. There was a skylight above their heads, and by the moonlight floating in through it they could see each other's faces. As they stepped onto the landing, they both said "seven" and passed by each other, and then started laughing together. He once again turned to face the bottom of the staircase.
The eighth step. The staircase descended down into darkness and the hallway that extended from the bottom of the stairs was dimly visible. The ninth step. The tenth step. He was going down while his friend was going up. After counting the eleventh step, the twelfth step, the thirteenth step, while feeling his way with the tips of his toes, he reached the bottom of the staircase.
He tried to call out that he had made it to the bottom, but he heard a voice say "fourteen" from up the stairs. Laughing to himself, he turned around and looked up. He expected to see his friend looking down at him laughing mischievously. But all he could see was the hand rail extending up into the darkness. He couldn't see his friend. Again the sound of a voice came down from above. "Fifteen." He also heard the sound of a soft footstep. The school was a two-story building, so there was no third floor. The voice continued: "sixteen," "seventeen." The voice now sounded like it was on the verge of tears.
He went up a few steps. The top of the staircase was still too dark to see. From the darkness, he heard a timid, far-away voice say "eighteen," He called back, telling his friend to turn around.
"Come back down!"
His friend didn't turn back. The voice was becoming more distant, counting helplessly. Holding his breath and standing in the middle of the staircase frozen with fear, he heard the thinning voice one last time before disappearing: "twenty two." And it wasn't just the voice that disappeared. He never saw his friend again after that.
The name of the boy who remained isn't known, but the last name of the boy who disappeared started with N.
There are other versions of this story where N-kun goes down the stairs. In some, the phenomenon happens while going up the staircase. In others it happens while going down.