It's in chapter 21 of To Kill a Mockingbirdthat the jury finds Tom Robinson guilty, despite convincing evidence presented by Atticus. In fact, it is likely that some members of the jury (or all) believed Tom was innocent but feared retribution from other jurors and from others in Maycomb, a town with an understood, at least residual, history of racism.
It is this tradition of racist attitudes which interferes with a fair judgment of Tom by his peers. In Chapter 23, Atticus says, "The one place where a man ought to get a square deal is in a courtroom, be he any color of the rainbow, but people have a way of carrying their resentments right into a jury box." Shortly before Atticus says this, he notes that if the jury had been filled with Jem and eleven other boys like him, Tom would be a free man. Jem is open minded on account of teachings from Atticus. But the other implication is that Jem is young enough not to be bogged down and brainwashed in the racist traditions of the town.
Even the sole holdout on the jury, one of the Cunninghams, eventually succumbed to peer pressure from other members of the jury. Tom did not get a fair trial by a jury of his peers because they relied on social tradition (which includes racist tendencies) rather than reason when determining the verdict. Not to mention, it might be wrong to label the jurists Tom's peers because a peer implies someone who is your equal. Clearly, Tom was not treated as an equal; another example of the impossibility of a fair trial.