After Jem hears the verdict, Scout notices that "his hands (are) white from gripping the balcony rail, and his shoulders jerk as if each 'guilty' is a separate stab between them." This implies that Jem is shocked, angry, and pained by the verdict. He has seen the trial unfold and was convinced that Tom Robinson would, on the basis of the evidence, be found innocent. The fact that his hands grip the balcony so tightly could suggest anger, as if he is trying to concentrate his anger through his grip rather than voicing it while the court is still in session. The simile comparing the guilty verdict to stab wounds also implies the pain and shock that constitutes Jem's initial reaction. This reaction also shows that there is a kind of violence to the verdict, or, more specifically, to the racism behind the verdict.
After their father leaves the courtroom, Jem's shock gives way to a cathartic release of sadness and grief, and he cries. His face is "streaked with angry tears" and he keeps muttering "It ain't right." The anger and the incredulity are indicative of Jem's realization that the law is not impartial but subject to the racism of the people it depends upon. This is a shocking and saddening realization for a child to have.