That is Scout's role in the Halloween play. (she was ham)
A ham is an actor who over-acts, or acts melodramatically instead of naturally. Thus, to ham it up means to over-act, to try to be the center of attention, or to be melodramatic.
Yes. Scout beats up Francis for calling Atticus a "Nigger-Lover" on Christmas
She is a part of her school show, which displays the products of Maycomb, one of which is ham.
Usually iambic pentameter. Rhyme scheme: abab/cdcd/efef/gg