What are the fist clues that piggy is a lower-class person?

Ralph pretty much ignores Piggy when they first meet. He finds Piggy an irrelevance. As the story continues, Piggy is seen as an outcast. The only moments all the boys come together in agreement is when they humiliate Piggy.

Shut up, Fatty.”
Laughter arose.
“He’s not Fatty,” cried Ralph, “his real name’s Piggy!”
“Piggy!” “Piggy!” “Oh, Piggy!”
A storm of laughter arose and even the tiniest child joined in. For the moment the boys were a closed circuit of sympathy with Piggy outside: he went very pink, bowed his head and cleaned his glasses again.

The ultimate irony is that we never find out Piggy's real name.