To find the key..
You gotta kill that funny looking ninja guy in the women's restroom. He yells at you a little, shuts off the lights, then dances around in the dark with his sword. Point and click in the vicinity of the shining red light a few thousand times and he dies. Then click his head and it kinda.. falls off.. grab the NVGs and head back to your first room. Use the NVGs, grab the key, then run back through the hallways, stopping at the one guy with the machine gun you killed earlier, just to pump a few more rounds into him. Then continue on to the teacher's lounge, use the key, jump through the door. Shoot the shit with a few more Goths, then a boss appears.
Good Luck