This happened when I was in sixth grade. My classroom had an another room attached to it (My school was a British king's place where [according to legend]his wife and daughter used to live). We weren't allowed to go inside but me and my friends always took a peek through the glass doors. It looked like a bathroom because it had an old fashioned mirror on a side, you could see pulled off pipes everywhere and even a filled pit which seemed like there was a bathtub there before. It even had two beautiful wood-carved cupboards with glass paned doors.
One day after school I was waiting for my friends to get our teacher's registers to keep them in the cupboard (I was the in-charge of the cupboards so I had the keys). We were about to leave when we heard a noise that sounded like a cupboard was flung open. When we turned we realized that the noise came from the attached room. So we decided to take a peek. We saw the silhouette a beautiful woman in a gown who seemed to be in her early twenties opening and closing the glass-paned cupboard again and again. Then she suddenly walked towards the other cupboard. We were busy looking at her when we heard a knock on the classroom door. We saw our headmistress standing near the door. Within a second we turned to the bathroom again and saw that the girl had vanished.
After that day I always went home as soon as the bell rang. My friends and I never peeked into the room again and never talked about it. But I still wanted to know more about her. So I looked on the internet and found that the girl we saw was the daughter of the British king and had died in the castle after suffering from a disease for a long time. I even asked my teacher (we were very close to each other) and she told me that my classroom including the attached room, was the princess's room itself.
Even though I was a little scared during this experience (as it was my first one) I wanted to see more and more ghosts. And I did have many more experiences after this.