PS: its easy if you go up NOT DOWN
1) Go upstairs
2) Find the line its near the 2 windows that are parallel above the box room (Thompson room I believe)
3) Follow the line to the square on the wall
4) crouch and enjoy...
another PS: If the zombies Pile infront of you, they may be able to hit you through the window next to you...
This does only work with the square closest to the sawed off shotty
Also, they will pile below you... so if your friend is down there, be careful