"How d'you expect Firenze to climb that ladder? We're in classroom eleven now, it was on the noticeboard yesterday."
-- Parvati Patil (OP27)
A classroom located down the corridor leading off the Entrance Hall on the side opposite the Great Hall. This is one of several unused classrooms on the ground floor. Dumbledore arranged for this room to look like the middle of a forest clearing; Firenze, who couldn’t easily negotiate the stairs, taught Divination in this room. He could lower the lights and display a starry sky with a wave of his hand. (OP27)
Classroom eleven, specifically, is only mentioned in OP27, though there are other mentions of other "unused classrooms" in the series. See PS12 and OP16.
"It looked like an unused classroom. The dark shapes of desks and chairs were piled against the walls, and there was an upturned wastepaper basket -- but propped against the wall facing him was something that didn't look as if it belonged there, something that looked as if someone had just put it there to keep it out of the way." -- PS12
"Maybe an unused classroom?" -- Dean Thomas (to Harry Potter) (OP16)