It is true she might break a window, but what good would that do?
She couldn't jump down three stories into a stone court below.

She went to the window and looked out.
"If I hung by this window sill," Grace said aloud, "I believe my feet would just reach the cornice of the second-story window." Seizing a heavy ruler from one of the desks, she ran to the window and deliberately smashed out all the plate glass in the lower sash.

Then, hoisting herself onto the sill, she looked down from what seemed to be rather a dizzy height.

But nerve and determination will accomplish anything, and Grace turned her eyes upward.
"I shall do it," she kept saying to herself over and over.
Clinging to the window sill, she gradually let herself down until her feet touched the top of the cornice underneath.

Then, steadying herself she looked down.