Where is the exit in ghost house 1 Super Mario World 2?

Enter the door near the checkpoint flag. This will take you to a room with moving green blocks and a coin roulette box. Wait along the right side of the room and eventually the moving green blocks will leave a gap in the right wall. Quickly jump through this gap and go through the door on the other side. This will take you to the secret exit flagpole.

This secret exit leads to the world 1 warp cannon which will take you to the mushroom world.