A classroom was a room in a building, usually a school, where classes were held.
In the final draft script of "Miri", this setting was, instead, a principal's office.
Prior to her assignment aboard Enterprise NX-01 in 2151, Hoshi Sato worked as a linguistic teacher in Brazil on Earth. She taught her class in an exterior classroom. (ENT: "Broken Bow")
By 2266, some of the Onlies on Miri hung out in an abandoned classroom which among other things featured a blackboard. (TOS: "Miri")
Aboard the Galaxy-class starship, there were several small schools of varying sizes located throughout the ship, ranging from actual classrooms (TNG: "When The Bough Breaks") to specialized workshops. (TNG: "Imaginary Friend", "Rascals", "Masks") The USS Enterprise-D had at least 8 schoolrooms. (TNG: "Rascals")
Simon Tarses declined to become a Starfleet officer because he had no desire to spend four years in classrooms when he could be traveling in space instead. (TNG: "The Drumhead")
Keiko O'Brien opened a school on Deep Space 9 in 2369. To aid her in this purpose, her husband converted a room on the Promenade into a classroom. (DS9: "A Man Alone")
DS9's classroom was damaged by a bomb later that year, after Vedek Winn Adami started protesting against the secular subjects taught by Keiko. (DS9: "In the Hands of the Prophets")