Q: Why did the bat miss the school bus?
A: He hung around too long!

Explanation: First of all, let’s be clear: Bats don’t go to school.  Although, bats might make their home in the attic of a school or any building if there is a way to get inside.

In this joke, when you miss the school bus, it means that you arrived too late to take the bus.  There are other meanings of miss the bus: If you really liked taking the bus and you no longer take it, you might miss the bus, just like you might miss someone who is not with you; if you were trying to hit the bus with a snowball (don’t do this!), you could miss the bus when you throw the snowball and the snowball does not hit the bus.

To hang around means that you spend time with people, maybe talking, maybe not doing much of anything.  If you hang around too long, that means that you are spending too much time at a place and you will be late for whatever comes next.  Bats, of course, hang upside down when they sleep.  That is, they grab a branch from a tree, or something else, with their feet, and they hang with their heads toward the ground (the opposite of standing on your feet).

This joke is funny because it plays with the words hang (hung in the past), like a bat sleeping, and hang around, like spending time not doing much of anything.

Here is a short video about bats landing upside down: