Q: Why did the teacher wear sunglasses during class?
A: Because her students were so bright!

Explanation: Have you ever seen a teacher wear sunglasses while teaching in a classroom?  I haven’t.  Most people only wear sunglasses when they are outside in the sun.  The light from the sun is so bright that it is good for your eyes to shade them with sunglasses.

Students who are smart can also be called bright.  In this case, smart and bright are synonyms.

This joke is funny because it plays with two meanings of the word bright: giving off lots of light and smart.

Here is a song about a bright future and the use of sunglasses (shades):