A classroom aboard the USS Enterprise-D.
A classroom is a place where children are taught.
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")
Keiko O'Brien opened a school on Deep Space 9 in 2369. Early students included Jake Sisko, Nog and Luta. (DS9: "A Man Alone")
Several weeks later, Keiko went on a trip to Earth, leaving Miles in her place as a substitute teacher. It was while he was doing so that he advised Benjamin Sisko to separate Jake from Nog, telling him that the Ferengi youth was a bad influence. (DS9: "The Nagus")
After Grand Nagus Zek expressed dismay that Nog attended as school run by a human female, Rom briefly withdrew Nog from the school. (DS9: "The Nagus")
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")