The Boy Running In School
I used to be a school teacher for the sixth-grade kid's many moons ago. My classroom was close to a boy's restroom. One time, I was done for the day and looking forward to the weekend. It was definitely a Friday because we didn't have after school activities, and no kids could hang out at the school. This is important.
As I was leaving, the teacher next door and I started having a conversation in the hallway, just catching up and planning for the following week. Suddenly, she says to me "look, there's a kid there." I turn around and see a kid of about 8 years of age standing by the door to the restroom. I remember he was wearing a yellow T-shirt with jeans and white sneakers. He looked light-skinned and had brownish hair. When he saw us looking at him, he ran inside the restroom. He opened the door and went in. This could also be important.
My friend asked me to go in there and get him, in case he was the son of one of our fellow teachers. There was another elementary campus across the street from ours, so we thought he might also have come from there (the yellow shirt looked like the uniform those students would wear). At any rate, we knew we couldn't just let him stay there unsupervised.
The restroom was built in a way that I had to walk around a partition to get to the facilities area. The kid wasn't visible, so I thought he probably had hidden in a stall, even though I couldn't see anyone's feet either. I called out to see if he responded, and he didn't. I knocked in all of them, but there still wasn't any answer. At this point, I'm more upset than scared because I wanted to leave, so I pushed open the doors to all the stalls (none of them were fully closed or locked). Nobody was in any of them ...
I figured, "OK, somehow this kid sneaked out past me."
As I was walking out, I heard a faucet in a sink try to open, but there was no one there.
I walked out feeling creeped out. My friend asked me about the kid, and I said "he's not in there." She looked surprised, and I asked her if she had seen him sneak out past me. She said no one had walked in or out since I had. I asked her to go in and look for herself. She went in and gave the place a quick look, saying there was no one there after she came out.
We gave each other a look and decided to leave for the day ...
To this day, I really can't explain if a young kid really outsmarted us, or if we really did see a ghostly apparition. 
As a final note, that building was very old, and there were stories of ghost sightings. One of the night time security guards described seeing a kid like the one my coworker and I had seen some time after we did ...