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!).