Monday morning at our high school smelled of wet chalk and freshly brewed tea. The clock tower had been closed for years, seemingly too fragile after renovations. At 7:17, however, I heard the bell, clear and cool as a drop on the glass. It shouldn't be working, said the regulations, and yet the walls vibrated as if waking up. I'm Lena from second C, the newspaper editor, so strange sounds are my element. - Did you hear that? - I asked Kuba at the radio station, where he was testing a new mixer. He just nodded and played a recording of the morning's sound monitoring in the corridors. We glanced at the graph; the waveform was perfect, as if someone was calling from inside the wall. A minute later came an anonymous message to the paper: 'The tower is not empty', with a plan of the north wing. Deputy Director Nowak immediately sent an announcement that the installation had failed, although no one had touched the tower. In the library, Ms Irena pulled out the school chronicle, thick as a brick and smelling of dust. Between pictures of classrooms from the 1990s, someone had written in ink: "Do not open 312 at dusk". Under the table I found an old key ring with the number 312 stuck to the tabletop with tape. - 'There's no room like this anymore,' muttered usher Felix, passing us a mop and avoiding our gaze. - Time goes differently in this wing, and doors like to remember other people's keys. Kuba looked through the book of school keys and muttered that the number three hundred was chalked with a red marker. After lessons, we walked behind the auditorium stage, where an old curtain hid the entrance to the north corridor. The lamps blinked nervously and the tiles underfoot creaked like an unworn violin. On one doorframe a trace of numbers could be seen under the paint, clearly three, one and two. I tried on the key; it fitted as if it had been waiting for us since the last bell rang years ago. - 'If it's a stupid joke, we're going straight to the management,' warned Kuba, but he didn't withdraw his hand. The phone in his pocket turned on the torch itself, as if thirsting for anything we shouldn't see. The lock vibrated and the door opened a centimetre, letting out a plume of cold old air. Inside flashed something like a projection, although no one had turned on the electricity in this part of the building. A new inscription appeared on the chalkboard, damp and sharp, like a fresh whisper. There was my name and the exact time, equal to the second we stood in the threshold. Before I had time to move my hand away from the door handle, the bell struck a second time, even closer. Dust danced on the air and formed into a shape we both recognised from the corridors.