Here is a story told to me by my school's 6th year form master.
First I'll give the background - my school is very old (going back to the 1800s), and in it's time, it has quite a bit of history. For obvious reasons, I can't mention my schools name, but for the purposes of the story, I'll assume what I've been told by many people to be true.
In the early 1900s, a man was murdered somewhere in the 6th year corridor under suspicious circumstances. There's official documentation to prove that someone was murdered at the school at that time in the 1900s, but one of my friends has told me that in the 1950s, a mad friar burned down the archives that kept the information. Our school is run by a religious order, but they're quite secretive about the murder - one of my friends asked one of the friars, who reluctantly confirmed. A different friar told me that the murder never happened. There are many other rumours about my school, but I would be going off on a tangent to explore them.
To describe the geography of my school, it has a lot of fields, with the school buildings in the centre. In the centre of the main building is a square open air courtyard, and one side are the 1st and 2nd year classrooms. The other 3 sides are passages with various rooms on the side. Up one storey are the 3rd and 4th year classroom, the labs, the 5th year classrooms and some more rooms. Finally, the 2nd storey is one above that, and this contains the 6th year corridor, where this murder took place. The 6th year corridor is very quirkily shaped (imagine a question mark with corners where the curves would be). One of the rooms is now sealed off (this was for practicality reasons - it used to be a common room when my school used to be a boarding school, but it now has no use. All that remains is a door that leads to nowhere). Behind that door, the man was murdered over a century ago.
The 6th year form master in my school told me that he was up there one night at 9pm, and he unlocked one of the doors to the 6th year corridor (the doors to the corridor are all locked at night time). He locked the door after himself, meaning that no one could have followed him. He switched on the light and began to walk along the corridor, and while passing the classroom with the sealed door at the back of it, he looked in at the door, and then back at the corridor, he saw a man standing still. He said that this man was wearing a waistcoat, had trousers down to his knees, a bow tie and shoulder length straight hair that was combed back. The 6th year form master could see approximately the side profile of the man, so he could see some of his face, and he said that he looked quite depressed. Being a bit creeped out by the man standing still and solemn, he hurried along to his office and when he had collected the stuff in his office, the man had disappeared.
When I hear a ghost story, I tend to keep an open mind, but at the same time, I'll try and find a rational explanation. I definitely don't believe my teacher would lie to me; I trust him fully, and would say that he was being honest with me. I highly doubt that someone hid in the corridor and dressed up in old fashioned clothes just to trick the form-master, and anyway, no-one in my school looks quite like the man my teacher saw. The only realistic explanation is that my teacher imagined the figure, but he's a normal, level-headed teacher, so I can't see a reason why he would imagine the man.
My school is really a fascinating place, and there are lots of rumours surrounding it, especially the attic (quite a scary place I'm told), however, this story is the only one I've heard that I find believable.