Question

Why does Percy think the whole school is playing a trick on him

Answer 1

I'm assuming you're talking about the story "Lightning Thief"  or "Percy Jackson" since you forgot to put the story name , but that's okay though! :) 

But, personally I think your answer, would  every single time Percy would ask about his teacher, Ms. Dodds people would look at him like he was a crazy person because  the people don't know who "Ms. Dodds" is 

Good luck on your assignment and enjoy your day! 

                         ~$MeIsKaitlyn:)$

Answer 2

Percy thinks that the whole school is playing a trick on him because Mrs. Dodds disappears and no one seems to know who she was. By the way, if you've only read that one book, you should read the rest of the Percy Jackson series! I loved them!