Why doesn't Mayella confess the truth? Consider the circumstances of her life and the social context in which she must continue to live.this was in...

Think about the racial dynamics of those days and think about what her father is like...

One of the reasons that Tom is in so much trouble is that black men and white women are not supposed to have anything to do with each other in that time and place.  So if Mayella has been voluntarily doing something even a bit physical with a black man, she will be totally scorned and shunned by other whites.

In addition, her dad is a pretty violent man.  If he feels like she has humiliated him, he will surely beat her.