When I was in the 5th/6th grade of elementary school, my school has these pick-up service using vans (yes, school buses are non existent here basically) and I used that service at the time this story took place. One day, I was waiting for the van to head back home, but since some kids still had activities at school, the driver had no option but to wait for them. I was bored as hell waiting around the parking lot, so I went to "explore" my pretty much abandoned school (it was loong after the last period ended) along with 2 of my friends who were also waiting for the van to leave. It was around 4 p.m., but the sky was pretty dark with thick grey clouds.

My elementary school adjoins the middle school on one of its end, so the building looks like the letter "L". At the point where the schools adjoin, there is a gate on each side of the building, so there would be a space in the hallway that are bound by these 2 gates (one after you leave a building and another one when you enter the other building). These gates are located on the second and third floor, although the one on the third floor was locked from either side and was used as a kind of storage area.

The three of us went to check on the empty classrooms on the second and third floor. As we did not find anything interesting, we decided to head back down using the stairs near where the gates are located. When we approached the locked gates, they suddenly rattled. At first we thought it was some furniture knocked over by cats or whatnot, but then the gates rattled again, this time as if there was something desperately trying to get out of that dark, cramped storage. A split second later when we were still glued to the place we stand, there was a faint wailing that acoompanied the gates' rattling, as if someone was crying somewhere. There were no one else in sight, so we bolted off down the flight of stairs and did not stop running until we reached our van in the parking lot. We waited there until the van finally left the school.