First off, I'd like to begin by saying that this isn't an experience of my own. This happened to my mother when she was around the age of 11. It all started when my mother and uncle were sent to boarding schools across the country. My uncle was to attend a private school in the countryside and my mother was sent to a much older and more disciplined boarding school near Maidstone,England. One of the rules in the school was that you should always knock before entering the bathroom. This was due to the fact that there weren't any locks on the bathroom doors.
One night as my mother was getting ready for bed she decided to take a late bath. After knocking on the downstairs bathroom door and receiving no answer, she enters. Upon entering my mother remembers seeing a girl laying in a bathtub full of water. The girl was facing the wall so my mother couldn't make out who it was. She just saw a girl with pale, almost blue skin and long golden hair that dragged on the floor behind her. My mother apologized for disturbing the girl and quickly left the room.
Keep in mind that in a boarding school with around 300 students at the most, everyone knows everyone. Even if she didn't know someone personally, chances were she had seen them around before. My mother however, couldn't think who the girl in the bathtub was, because nobody in the school matched the appearance of the bathtub girl.
Another girl my mother knew who in this story we'll call Jane came downstairs. After asking my mother if the bathroom was occupied Jane was also surprised to hear the description my mother gave about the girl inside. Jane gave an unbelieving look and asked if my mother was sure, though my mother didn't usually lie or tell tales about anything. Just to be certain Jane knocked on the door and once again there was no answer. Though as they entered this time, there was no one in the bathroom. The girl had gone. Jane laughed it off, but my mother remained very serious about the whole situation.
The next morning my mother took it upon herself to investigate a little bit. She asked one of the head teachers if there were any new students in the school. After receiving a negative answer, my mother was very confused. The teacher questioned my her and asked why she thought so. After my mother retold the story to the teacher, she looked confused too for a while... But then her face turned pale. The teacher just sat there for a while before telling my mother the real reason there were no locks on the bathrooms.
Years ago, many girls would get so upset and depressed that they would lock themselves in the bathrooms for hours. Most of the time no harm would come to the girls. But there was one tragic event that changed that... There was one girl who decided to take a late bath before bedtime. She entered the bathroom and locked the door. The girl fell asleep in the bathtub and supposedly drowned. Nobody was aware she was in there. She was found the next day lying in the bathtub, facing the wall, the bathtub filled with water.
My mother to this day swears by what she saw and to this day she is a strong believer of the supernatural world and all it has to offer. I also believe her.