The National Police say the suspect was carrying the fatal gun when arrested.
Spokesman Donald Gonzalez says the boy told police he was paid to shoot the woman as she took her two children to school on the outskirts of Guatemala City.
Interior Minister Carlos Menocal said Thursday that street gangs hire minors to carry out killings because they face lighter treatment in Guatemala’s legal system. The boy could face 6 years in a juvenile custody facility if convicted.
The legislature is debating a law that would lower the age at which suspects could be tried as adults from 18 to 15.