I think Holden think others are phony because he dislike Pencey prep, so he may dislike the student too. Because Holden thought the school’s ads are fake, so he may thought students on Pencey prep also phony. Actually, I think Holden is a little bit fake and hypocritical because he told lies to his teachers. He didn’t follow what he had said. And he admit he is a liar himself.
While Holden was talking to old spence, Holden said he was going to get his equipment. But it was a Lie and he just used it to leave old spence’s home. While Holden was fighting with his dorm mates and his dorm mate asked him to stop cursing at him. Holden agreed, but he kept cursing his room mate. So that's why I think Holden is hypocritical and phony.