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

The other students refuse to admit Mrs. Dodds existed.

The students acted as if they were completely and totally convinced that Mrs. Kerr—a perky blond woman whom I'd never seen in my life until she got on our bus at the end of the field trip—had been our pre-algebra teacher since Christmas. Every so often I would spring a Mrs. Dodds reference on somebody, just to see if I could trip them up, but they would stare at me like I was psycho.

It got so I almost believed them—Mrs. Dodds had never existed.