If she doesn't she is sure to jump.

Betty! Betty!" she called, as loud as she dared, but the next moment the door opened and Aunt Pike entered with a candle in her hand, and followed by Anna.
"Dear, dear," she said, as she tripped over something, "how untidy! What is it, Anna ?" Anna stooped and picked up one of Betty's discarded gray stockings.
For once Betty's untidiness served them a good turn.

Seeing the stockings on the floor, it never occurred to Aunt Pike but that they had both undressed and got into bed in the usual fashion.

The first thing, though, that caught Anna's eye was the red bow in Kitty's hair.
"I--I didn't know--" she began, then glanced quickly at Betty's head, where the blue bow showed up against the pillow, but instead of remarking on it she suddenly grew silent.

Kitty marvelled, for she had remembered their hair ribbons almost at the same moment as Anna had caught sight of them, and it was all she could do not to put up her hand and grab hers off.