My experiences centred around the time I was in junior school about 20 years ago. I loved my school but a lot of stories went around the school about the large school stage being haunted. The stage was a large platform (with a small hatch near the front) with curtains either side which led to a corridor behind the stage. There are three doors in that corridor which are small storerooms. The stories were of a young boy that went missing and the bones of a child (presumably of this boy) were found inside the small hatch. Not long after an old headmaster had supposedly hung himself behind the stage and him and the boy haunt the place.
During one Christmas concert when my class had to all stand on the stage, a friend of mine was stood next to the curtain that was partially open. She was seen crying and terrified while she stood there and later told us that something white repeatedly flowed past her disappearing behind the curtain.
Now, during break times and lunch times no one was allowed inside the school. Teachers very strict about this and always guarded the doors so no one could sneak in and go messing around. Teachers were also in the halls keeping watch. One lunch time I was bored and I begged the main dinner lady to let me go and get a ball from the games cupboard in the hall. She allowed me and kept watch just inside the entrance door while she waited for me. I ran into the big empty hall and quickly grabbed a tennis ball from the cupboard. As I came out I stopped dead as a banging sound came from somewhere behind the stage. It gradually got louder as it sounded like it was getting closer to the front. The curtains began to move and flap about, flicking up wildly as the banging got louder. There was no one flapping the curtain around. I dropped the ball and ran.
There is a small set of steps at the side of the building leading to the fire exit door for the corridor behind the stage. There is a small window next to the door allowing you to see all the way down the corridor. Me and my friends would hang out and sit against the door. One afternoon I was sitting with my back against the door lightly tapping on it with my knuckles. We all jumped as there was a loud knock that came from the other side. We jumped up and looked through the window to see if anyone was messing around in there. If there was anyone there they would be seen as there was nowhere to hide and would be seen running down the corridor. There was no one there.
Very high up on the side of the building near the roof of the stage there is a circular window. It never gets opened as it like 50 feet high. Many kids claimed to see a face of a boy looking out of this window. I was very sceptical until one break time I was sitting on the grass and looked up to see what was indeed a face of a boy looking out of the window. I kept staring until the bell went. After that no one seen or experienced anything again, at least not to our knowledge.