Back in the summer of 1999 I was in secondary school.
On this particular day our final lesson was PE and afterwards myself and 3 friends stayed behind a little after everyone else to help put away the equipment, so by the time we went to shower and get changed everyone else had left.
The PE hall was at the bottom end of a long straight corridor and so you could easily see up and down it and see who was coming and going etc.
We finished showering and were walking back up the corridor, just myself and my friends.
Nobody else was around as it was quite a while after the end of the school day by this point, and so most of the kids had already left for home.
We were just slowly walking along, chatting and basically taking our time, when suddenly, a boy of around 8-9 years old pushed past us from in front, we physically felt him barge by our shoulders and all of us spun around to see who it was - but there was nobody there!
At first I assumed only I had noticed him, until one of my friends looked at me and asked "Did a little lad just push past you?"
To which I nodded yes.
The corridor had no doors along it and nowhere this boy could have possibly gone in the millisecond it took for us to look around.
Also, we all realized none of us had seen him approaching, only noticing him as he pushed past us before vanishing again, and the length of this corridor meant that we would have definitely noticed anyone approaching from in front of us.
The school in question is built right beside a cemetery, and likely on top of part of it too.