What are the first intimations that Piggy is a rather lower-class person?

Our first clue would be the use of the word "was." Only someone in the "lower" class would have spoken using this dialect (not using proper English).

"We was attacked!"

"He’ll be back all right."

The fat boy shook his head.

"When we was coming down I looked through one of them windows. I saw the other part of the plane. There were fames coming out of it."