The joke:
I went to school for magicians but failed the final exam.
They were all trick questions.
The joke plays on the double meaning of the word trick.
- In a magician school, a “trick” is a magic trick (what magicians perform).
- In normal speech, a “trick question” is a confusing or misleading question designed to fool you.
So when the person says the exam had “trick questions,” it sounds like a normal complaint about a hard test, but in a magician school it also literally means the questions were about magic tricks. The humor comes from that double meaning.