While the floor is not carpeted, it can still transfer charges to you when you walk on it. All objects hold onto charges to some degree. If two objects rub together, and one holds charges more strongly, then there is going to be a static build up. A bigger difference in the objects' hold on their charges means greater charge build up.
In your case, the shoe rubber and the tile have different enough grasps of their charges that when your shoes rub the ground as you walk charges build up. Dry air in the room could also contribute to the build up of charges (since charges will dissipate off an object more slowly in drier air).
And, of course, when you touch the large metal conductor on the chalkboard, the charges run off and you feel a shock.
So, while the mystery of the 'math class shocks' is gone, there is still a little suprise waiting for you everytime you go to the chalkboard. :-)
(republished on 07/13/06)