I had to look it up pan, and it's the opening lines of Tom Sawyer & Huck.
orister - it's a dialect/accent type of thing.
Like someone could say "Oh Law!" and mean "Oh Lord"
Twain was trying to spell the word the way the teacher would have said it.
If I'm being casual, instead of typing "going", I might type goin'.
You're right in your interpretation I think. Tom is a rascal, a troublemaker.
Saying "what's gone w/ that boy" maybe rhymes with "what's wrong with that boy"
Maybe Twain was also making a play on words.