After leaving the discussion room, go to the square and turn right first. You can get a stone sword key on the tower in the center of the square and move on.
Come to the right of the square, cross Liang Shi and reach the Broken Bridge.
On the way forward, the iron ball will fall ahead. Move along the left side first, and move to the right side immediately after the iron ball hits the ground to avoid the iron ball.
After entering the building, the portal on the right terrace will transport us to Jieyuantang and then to the library.