"It's just ridiculous", Johnson said.
The girls were hospitalized with non-life-threatening wounds. Fire Department spokesman Larry Langford says he thinks both girls are students at the school. "There is no person in Chicago who is not disgusted by this incident".
One of the children was shot in the hand.
As they were leaving, shots were fired at them but missed, hitting the two girls instead.
Police said "persons of interest" were being questioned.
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They were also charged with one misdemeanor count of criminal trespass to vehicles, authorities said.
According to reports, the children, ages 7 and 13, ran inside the school after hearing gunshots.
The school has about 300 students in pre-K through 8th grade. Chicago Public Schools are still in session.
The Chicago school year ends Tuesday.
Police say three people were taken into custody after they were stopped in a vehicle linked to the shooting.
The shooting happened around 1:45 p.m.