Q: Why did the music teacher need a ladder?
A: To reach the high notes.

Explanation:  Ahh, yes!  The high notes!  When one goes up, like in an airplane or up a ladder, one is going up higher.  The child cannot reach the cookies because they are up high.  High notes are the ones more to the right on a piano; in science the larger the Hz the higher the note.  But high notes are not up high; that’s why this joke is funny!