Q: Why didn’t the skeleton want to go to school?
A: His heart wasn’t in it.
Explanation: A skeleton is just your bones; nothing on the inside and nothing on the outside. That means that there is no heart in a skeleton.
If your “heart isn’t in it” that means that you do not want to do something: Peter’s dad wanted him to play football but Peter’s heart wasn’t in it (he did not want to play football).
This joke is funny because of the double meaning: a skeleton does not have a heart and this skeleton did not want to go to school.
Here is a skeleton who was missing a bone (and a heart!).