Classism is prejudice against a group because of social class. An example is people’s preference for Socs over greasers.
The Socs are “the Socials, the jet set, the West side rich kids” (ch 1). The way society looks at rich kids is different from the way it looks at poor ones.
[The Socs] jump greasers and wreck houses and throw beer blasts for kicks, and get editorials in the paper for being a public disgrace one day and an asset to society the next. (ch 1)
When a Soc makes trouble, he is being a public disgrace but not a hoodlum. He will get fingers wagged at him and shaking heads, but nothing more. Greasers, on the other hand, are considered the dregs of society. They are the ones who are arrested, sent to reform schools and jails, and seen as frightening by the public.
This is an example of the double standard. From Ponyboy’s point of view, it is actually the Socs who are jumping the greasers, not the other way around. The Socs are the dangerous ones. The gresers are defending themselves.