|the structure is well considered and nice, though the plot is slightly overused, not that you can do much about that, I'm afraid. Now to get down to the little things you could change to improve the piece.
I'll go with the subjective first:
the language is a bit boring, probably could've used a thesaurus and the fact that caz didn't know her boyfriend of two years last name immediately pangs of false tension to me.
the objective problems:
just a few grammatical issues,
"Head down I walk the halls, turning this way, counting the doors, thing about Mark."
first off, pick one description of walking down the halls, please don't try to fuse them, second off, did you mean "And start to think about mark."?
"I don't know, don't care; just want them to go away before I end up making even more of a spectacle of myself."
place an I before "just" and you're golden.