I was in 6th standard at the time when this took place. I was of the firm belief that ghosts do not exist. In fact, even after this experience, I maintain the same. I still think that there has to be a sane and scientific explanation for whatever happened that day.
Our school is very old and there is a large forested area behind the playground. At that time, the basketball courts were situated on the border of that same area.
It was the end of school and we were all playing. We had just stopped for a break when the most unearthly sound filled the air, as if someone was singing. The closest comparison would be opera, but at a higher pitch. My hair stood on end. Everyone was looking around at each other.
There was no place around the court where the sound could have come from. The staff quarters behind the forest were abandoned. So was the guard house nearby. Besides, the sound was unearthly. No living organism could have emitted that sound.
There were just two people who had not heard the sound. But they did not believe in anything that was unexplained or mysterious anyway, not even things like the Hope Diamond or even some parts of history. This could be the only explanation why they did not hear the sound, considering the thing to be something supernatural.
We were all wondering what might the sound be. That was when a friend said that the sound might belong to a dear teacher of ours who died when we were in 4th standard. She died in a car accident, hardly peaceful. The children of her class had gathered whatever reminded them of her and buried it a little inside the forested area, right in front of where we were standing. This could be proof that the sound indeed belonged to the teacher only.
Until I find a reasonable explanation, I will keep remembering the day as the day when my deep seated beliefs were challenged.