Well, I'm disappointed to see that not many people have really explored the replay chapters for secrets, Grudgie... I was hoping to get an answer to that, too.
I'm pretty sure the Shadow Man is just there to scare us, but it really was an interesting find! After thinking about it for a while, I came up with this:
Maybe the Shadow Man is actually a former teacher at the school, and the heartbeats we heard weren't actually heartbeats, but heavy footsteps...
My reason for this conclusion? You left out a little bit of information, Grudgie:
upon entering the classroom the Shadow Man was located in, we stood in the doorway when we saw where the black figure was standing. He was standing near a student's desk!
I'm not sure if this is important, but I also recall another detail. The desk that the Shadow Man was standing next to was one closest to the windows, and had a red handbag hanging from its side.
Maybe he was a teacher who liked to walk around the classroom, making sure that students were doing their work?