They had a box cutter,crowbar and what looked like a shiv.
Religion is regarded by the common people as true, by the wise as false, and by rulers as useful.

it was a crow bar not a fire extinguisher

plus the cut her tendons and other parts of her up.

Teacher in the office was a fire extinguisher, I assume the girl in the locker room was a crow bar as you saw one of the boys holding it in a close up.

This is very true. In reality, a group of teenage boys could indeed smash a girl's face to pieces if they wanted to.

Yeh, I'm no expert of course, but I would imagine it would be fairly easy to knock someone's teeth out and break their jaw with a heavy blunt object.

Hit the crap out of her with it, just smashing away the face. (doesn't actually show it though, we just assume)

How do they use it?
Why does Snoop Dogg use an umbrella.
Fo' Drizzle.

They didn't, they used a
fire extinguisher