I can't get what they are laughing about, I know that it has something to do with the expression 帰りですかて but why? What is wrong/strange about the way Nobuko answered the question or is it the way the question was made by Ms Yoshimine?
Because Nobuko used the honorifics お帰りです for her own action. As you know, the speaker can't use honorifics for their in-group members including themselves. She should have answered to the question as ええ、あした帰ります instead. That's why they laughed.