Jem chooses to align himself with adults instead of children, telling Atticus that Dill has sneaked into the Finch house, having run away from home.
This is seen as a betrayal of a code of solidarity and takes place one night after Scout and Jem have been arguing and fighting. Scout feels that Jem has lost his sense of where he fits in the family, giving her orders that she resents.
Jem tells Scout if she antagonizes their aunt, he will spank her.
When the two are broken up, Scout goes back to her bedroom and thinks there is a snake under the bed. She calls Jem for help. They discover Dill hiding under the bed, hungry and dirty. Jem decides that the right thing to do - the responsible thing - is to inform Atticus of Dill's arrival.
Jem is behaving as a responsible adult by bringing Atticus into the room to show him that Dill is there.
By choosing to be loyal to a code of conduct that leads him to reveal Dill's secret presence instead of remaining silent, Jem has broken ranks with the children, Scout and Dill. He has demonstrated an allegiance with the adults.