Strong work -- here's what I suggest:
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Ring the bell and go right back up to the top walkway above the boiler. Don't go across too fast, or he'll see you from the right-hand side as he walks in. As he passes underneath you, sneak back across the walkway toward the door you came in. When the timing is right with the smoke, leap down from the edge of the walkway to the ground and land in a crouch. Then crouch-walk your way toward the lockers and run out. If you made it past the boiler on the way in, you'll be able to do this, too!