On Morris avenue in Modesto, CA is a building that has served as an elementary school and a youth center for nearly six decades before being damaged in a fire. Built in 1923, the building originally served as the John Muir Elementary School (and can boast as one of it's pupils a young George Lucas), and was later the John Muir Youth Center.
Local legend holds that people working in the basement of the old school building often hear the sounds of voices and dozens (or even hundreds) of feet pounding the floors overhead, as if children are running to or from class. Some sources even claim that this happens between 2:30 and 3:00 PM, the time when the children would have been let out of class for the day. Running upstairs to see what is going on, the workers always find the place empty.
In the obligatory "true ghost story" article that appeared in the 2007 edition of the Modesto Bee, someone claiming to be a city worker (but wishing their name to be withheld) claims that this very thing happened to her.
Commentary: In ghost hunter and paranormal enthusiast circles, this is what would be labelled as a "residual haunting", a more-or-less prosaic replaying of events that is creepy not in it's content, but in that it is happening without a clear source.
Despite having grown up in the area, I am not familiar with this particular building (I lived far enough away in the boonies that a youth Center on Morris Avenue would have been about as useful to me as a youth center on Mars). The first thing that I wonder on hearing the story is what the conditions in the basement are like. I have been in basements where the sound and vibration of a truck passing by outside sounds a lot like people running in the building overhead, and the echoes of people walking by outside sounds like people talking in the building above. Having had those experiences, I immediately wonder if something similar was happening in the John Muir building.
Another thing I would wonder is whether the stories were passed on by city workers who used them to freak out their more gullible co-workers. I have worked with people who do such things before, so it's not much of a stretch for me to imagine that happening as well.
Sources: Internet, Newspaper, Local Folklore