The African boy enters through the window (around 27 minutes 10 seconds) where he had placed the bottle cap, then goes to the door and unbolts the slide latch to the classroom. There is only 1 door in or out of the room meaning that whomever slid the slide closed must have exited through the window. You can see the room larger later when he is dancing. The other side of the room has the teachers desk and the chalk board, no door. There is no other lock on the door, but there is a knob, and obviously if the other kids can get to the outside of the door then it must open to the outside, thus a padlock would have been on the outside of the door, this makes for a big goof.