Mysterious Teachers' Lounge

The Teacher's Lounge is for teachers, Willie.

—The Simpsons, Superintendent Chalmers to Groundskeeper Willie.

In schools, the teacher's lounge is essentially a break room: A place for the faculty to enjoy time away from their duties as teachers of society's next generation. But from the students' perspective, the only thing they know about the Mysterious Teachers' Lounge is ... well, that they don't know what exactly lies beyond that locked door.

So in school settings, expect to see characters occasionally discuss and debate the possibilities of what might be inside. If enough characters get involved, they may even plan to actively break in and discover for themselves what sorts of secrets are hidden therein.

In more faculty-centric shows, someone may make an issue out of use of the teachers' lounge by support staff; in real life it's usually officially labeled as "Faculty/Staff Lounge" or some such even if nobody calls it that, for that reason.

Fun fact for today's kids: During the years between teachers not being allowed to smoke in the classroom, and teachers not being allowed to smoke in the school at all, the teachers' lounge reeked of cigs, and the smell was noticeable well into the hallway leading past it.

Compare Wondrous Ladies' Room, a similar place in which no man has gone before.

Examples of Mysterious Teachers' Lounge include:

Comic Books

In the "Teachers United" strip in the British comic School Fun, the teachers have an Elaborate Underground Base accessed through a 'Staff Only' cupboard.

Literature

In the Wayside School books, the teacher's lounge is referred to as a secret place where students shouldn't come.
Features in the Jennings books. In Thanks to Jennings, owing to a sitcom-style misunderstanding, the boys become convinced that the teachers go up to the room after dinner in order to secretly snack on a second course.
In The Best School Year Ever by Barbara Robinson, one kid starts a rumor that any kid who gets into the teachers' room will never come out again; this leads to everyone being terrified of the teachers' room and refusing to go anywhere near it.

Live Action TV

iCarly: "The Teachers Lounge", where Sam breaks in, and the icky gross teachers have affairs with each other.
Ned's Declassified School Survival Guide: When the door opened, dance music and disco lights could be seen and heard from the hallway along with smoke pouring out.
Everybody Hates Chris: the teachers lounge is a beatnik cafe where the teachers smoke, drink, and perform poetry.
Nick Hancock referred to this trope on Room101, and how disappointed he was when he actually became a teacher and learned that the only secret about the staffroom is that as soon as they enter it, all teachers become obsessed with biscuits to the exception of all else.
In Mr. D, Gerry argues that the school librarian shouldn't have access to the teachers' lounge. To be fair, said librarian tends to hog the microwave.

Web Comics

In Destiny Fails Us, Samson sneaks into the teacher's lounge just to change a few names around on his teacher's notes for the next project's study groups.

Western Animation

Recess: one episode involves this, and the gang expects it to be some sort of magical place, but is subverted when they actually see just being a normal room. Double Subverted when they leave it, and (I think) Principal Prickly presses a few buttons on the vending machine, revealing a hidden room with a spa.
A Pepper Ann comic in Disney Adventures uses this trope as the basis for its plot. When she is allowed to actually visit the lounge, her expectations on what it contains were crushed. After she leaves, the teachers reveal a secret luxurious room of the lounge.
On an episode of Arthur, we get a series of crazy imagine spots about what goes on in the lounge, complete with Lampshade Hanging: "And that's where our tax dollars go!"