What does jem do that shocks dill and scout?

Jem revealed the secret which he promised to keep and told Atticus that Dill run away and left home. This betrayal shocked Dill and Scout as he told about everything to an adult. Jem believed that a child can't run far away without its mother's knowing, and Scout says that Jem has already "broken the remaining code of our childhood." He acts more mature that other children and as a result he loses his friends.