A large collection of annoying misfits occupying the hallways of your high school. Typically they are sitting, running, kissing, listening to music directly in your path and are of general annoyance to everyone.
Hallway rats can easily be identified by overly baggy sweatshirts, clusters of people sitting and obstructing your path, anybody who has their headphone's volume cranked high enough so the whole hallway can hear them, two lovers who cant break apart making out in the hallway, or any group of white kids who hang out in the hallways and try to look gangster.
On my way to englsh class i had to carefully walk through a group of hallway rats who all decided to sit down in front of my classroom door.
Some scraggy looking hallway rat ran straight into me in the hallway and made me drop my books.
An annoying hallway rat was walking in front of me in the hallway with his music cranked up way too loud.