Very entertaining! I thought the name of your game was ingenious... and the voice was great - funny and sinister

I hope you got an A!
One minor thing - I thought the second voice where William stepped outside and realized all of his co-workers were gone - came a little late. I felt it was describing the empty room just outside his office, but that was spoken about after he was nearly out of that outside room.
As with all FPS Creator games the loading was slow, as was the frame-rate, as well as a few texture glitches - again all typical with FPSC. But it did better frame-rate-wise than most FPSC games I've played.
The lighting didn't look too real; what I liked about it though is the game had a more bright look to it, it wasn't so dark and grungy which has been way overdone to death. And the segment editor showed through in that the ceiling used the same texture as the floor. I don't know why so many of those segments repeat the floor for the ceiling. Some do, but it seems that most do not. A drop-ceiling texture would go really well with that floor and walls you chose. I liked your outside sceneries - I wanted to go exploring a bit, but I couldn't (walls).
I also found an invisible hole in the floor just before the door before the [basement ending] - there's an open gray box on the wall, if you walk toward it just before the door to the left, you fall through the floor and below the map to the red death of FPS Creator
And the 5 endings on level 1... don't you mean there are 4, but 2 ways to get to one? Or is that hole in the floor below the universe one of them?
I'd never heard of The Stanley Parable either. I checked out some play-throughs on youtube. Pretty neat game! I definitely noticed the smaller-scale recreation.
It would be cool if you expanded the game and added more of your unique flare to it (the uniqueness you already added was great!) - so that it stands even more apart from The Stanley Parable. I don't know if FPS Creator will let you do this, but you could add some randomness to what/when the voice is saying, to make each game unique, and thus likewise different paths would lead to different results in different games. In different games, some doors would be open, while others locked off, etc. Those are just some ideas.
Question though - some of the endings left you there, while others respawned the player. Is that right?
Edit:
I've realized that one of the endings I was thinking of for level 1 was actually a level 2 ending. So I only count 3 from level 1, unless you count the one where there's 2 ways to get to it, and the invisible hole in the floor.
I managed to get on top of one of the outdoor walls and peek over the trees. You built a huge landscape out there! No one would ever see it normally playing the game - love that realism!
I also managed to get into the rooms in the basement with the sealed doors.
It would be neat if it would be possible to survive the 2nd level shootout - forcing the voice to be wrong!

(as opposed to William just choosing a different way).
Another tip - You used all static objects, it'd be more realistic if you made them (or several of them) dynamic. Like chairs that would move, or vending machines that would vend, etc.