Here's my input. There were a lot of things I liked and a few things I didn't.
Things I especially liked:
The acting. Very well done by everyone.
The repeating steadycam shots going up the stairs looked very interesting, and made me wonder "who's walking up the stairs?" - I assume that was your intention so I think it worked.
The wide shot in the classroom, with the girl sitting along looked very eerie and set the mood. There are also a few other good still shots, such as when she walks down the hallway toward the camera, that kind of have a Japanese horror film style which I thought was well done.
The sound of the pumps was great.
The lighting was fairly good for indoors. I suppose you didn't prepare special lighting, but the natural lighting in the building worked well.
The shot from inside the elevator, when the doors open each time to reveal "Tang 9" was well done also. It made me wonder if something else was going to be revealed instead of the sign, since it was positioned to the left of the frame - a good way to reveal something.
The shot looking down the stairs (showing all the floors) was very cliche, but I thought it was useful here and I liked it.
Things I didn't like:
Some of the editing (sudden fades to black) and a few jumpcuts kind of took me out of the movie for a few seconds. I think with a few small tweaks this could be fixed.
Some of the sound isn't constant between shots; I can hear the sound start and stop for a few frames as the scene changes, and it was a little distracting.
The POV shot at around 8 mins in was kind of jittery (probably from slowing it down), and slow. The sound suggested she was moving quickly, but the video looked like she was walking down the stairs.
Some of the walking sounds aren't synced to her feet correctly. An example is at 8:15.
After the girl pushed herself, it cut to a shot of the mean girl, but it was kind of too quick, and I wasn't sure what happened. I had to watch it twice to understand that it was a different girl (a different version of the same girl). Perhaps you could've changed the camera's height to show that it's a different person (the same girl, but a different... uh, you know what I mean, lol).
Overall I really liked it
