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

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! 

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Cause it was important to him now stop being a bully