The underground classrooms are, as the name implies, classrooms that are below the Skool, possibly as a solution to the overcrowding in Skool.

Miss Bitters revealed their existence during the episode "Tak: the Hideous New Girl", as Rob was sent to an underground classroom so that Tak could have a place to sit. She also sent Brian down there because “she got tired of him”.

Since it was revealed in the episode “Lice” that there is a massive cavern below the Skool, it could be possible that the Lice Queen resided in what was the underground classrooms or what would become the classrooms.

It's unknown how long the underground classrooms have been in operation or if the students that are sent down here are allowed to leave, but it’s evident that they’re not common knowledge. as Dib asked Miss Bitters about them, only to receive an uncaring confirmation from her.

Despite the underground classrooms never making an appearance in the comics, in Issue 50, their presence could be felt as Miss Bitters sent a pair of students to the "Chamber of Worthiness", from which only one returns, freeing up a desk for Chammy Wamboo, which was pretty much the same purpose of the underground classrooms.

Facts of Doom

• It's unknown how deep the underground classrooms are, but presumably far enough below the surface of the Earth to ensure whoever is sent down there doesn’t escape.
• The underground classrooms have a similar purpose to the Irken education system, as in the unmade episode “The Trial”, it would have also been seen that smeets spend their childhood training bellow the surface of Irk until they are old enough to be considered adults.