"Well, they should be worried I thought. Twelve hormonal teenagers their lives in their hands. And I hold their lives in my hands. And a mad woman holds my life in her hands. Damnit, I hate this job."
I like the series a great deal, but I tend to like serial set ups. I'd cut to the important stuff by severe editing to let the repeat stand out. Also the first 'their lives' is ambiguous--I first took it to be the twelve themselves, not fellow students. I'd also seek out a rhythm this sentence structure can support. AHH, your sequence order is off, I think. hundreds to twelve to one.
Maybe:
"Hundreds of student lives
held by twelve homicidal teenagers, twelve teenagers lives
held in my hands, and my life
held in the hands of a mad woman. Damnit, I hate this job."
I'm finding the "Damn it, I hate this job." too over used. It would be fun to bring hands and lives back in, something like, "That's too many f****** hands and too many f****** lives. Why do I do this?" Well, depending on the age of your target audience. I'm YA, remember, but I'm thinking this is older audience for some reason?
OK, revision, revision, revision and now I've stepped over the editor boundary by adding in emotional content. How does this read to you, though?
"Hundreds of terrified student lives
held by twelve teenagers, twelve homicidal teenagers lives
held in my hands, and my freaking life
held in the hands of a mad woman. Damnit, I hate this job." And in this version--I like the "Damn it I hate this job..." :-)