The Shadow in the Music Room

It seems like the Seven Wonders of the World type of urban legends were popular when I was in elementary school. During the afternoon cleaning time, we were cleaning while telling ghost stories. The music room was on the first floor and faced the staff parking lot. 'It's scary if something happens in the music room, a typical school spot,' we said, laughing. As we were talking, a shadow suddenly slid across the wall. There were only two of us, including me, in the music room. I was dry-wiping the organ the students used, and the other person was sweeping trash in front of the trash can. So, there was no way a shadow could have stretched from one end of the music room to the other. I checked outside, but it was cleaning time, so there shouldn't have been any students or teachers in the staff parking lot. I still wonder what that shadow was.