A girl near the door smacks her gum and gazes up at the clock.
It says that a girl gazes up at the clock which is an easy guess to the reader that she doesn't want to do this or wants to move on. Which helps the image of them not wanting to to this anymore.
The kid in the Twins hat scrambles to borrow a pen from the girl sitting next to him.
This sentence adds a tiny detail of how the boy sleeping was unprepared for class. This adds more imagery to the classroom of how the students do not care much about the class.
He stares out the window that faces the street; it’s a still December night—no traffic, no sirens, no obnoxious noise or light. Across the street, a string of colored Christmas lights wrapped around a porch railing glow beneath a blanket of thick snow.
This section adds to the sadness of the classroom by telling the reader that it is December, with nothing much happening. December is also a month known for being a sad month.
In the back row a girl scribbles furiously in a fancy journal. Against the far wall a young man in sweat pants and a Minnesota Twins hat snores quietly—his desk empty, except for his head.
In this section it helps portray the fact that the students are bored.
Dana’s hand shoots up. She’s happy to be there.
These two sentences help show how Dana really wants to be there. Although someone already said this, it says shoots up which means she knew he was going to call on people, so she had an answer. It also says she "happy to be there" which even though it sounds bland it adds to the feeling of her wanting be there.