1) What is the headteacher actually saying?
When the author wrote this section, she wanted the reader to get to know the headteacher quickly.
2) What kind of person do you think he is?
3) How would it be different, if the headteacher had said this:
"Tyke, y' don't follow no rules and y' don't do any learnin' and you have a massive gob but even though I've known y' seven or eight years I don't reckon you lie."