An exterior classroom
A classroom aboard the USS Enterprise-D
The classroom aboard Deep Space 9
A classroom is a place where students are taught by a teacher.
Some of the onlies on Miri's planet hung out in an abandoned classroom. (TOS: "Miri")
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")
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. 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")