Southern California teacher Sondra Haile, 54, had her back turned when the two girls allegedly tipped poison into her soft drink, watched by a class of about 15 children. Before a lunch break, she was warned by another child what had happened and saw green crystals in the bottom of the drink.
Police questioned the children and say they quickly admitted what had happened. "You can call this a schoolyard prank, but this happened to be a deadly poison," said Dr Ronald Stephens, director of the National Schools Safety Centre in California. Prosecutors have said the dose was enough to kill Ms Haile. Two boys who hid the empty box of poison were charged as accessories.
The incident at the Lucerne Valley middle school stunned the quiet community of 10,000 people in the desert town of Lucerne Valley. The father of one of the two girls said his daughter was a cheerleader with a history of good grades.
It came on the heels of another shocking California case - attempted murder charges against a six-year-old boy in the San Francisco suburbs accused of severely beating a month-old baby.
But school violence aimed at teachers appears to be worsening, said Dr Stephens. "Students feel less reluctance to assault or intimidate someone in a position of authority," he said.
In a wide-scale survey, carried out for the Metropolitan Life Assurance Company, 11 per cent of teachers reported being assaulted, attacked, or otherwise intimidated in the course of one school year. Most serious attacks are beatings or shootings. This February, a 16-year-old boy in Washington state on the north-west coast of the US shot dead his teacher and two other pupils.
Prosecutors at first charged the two Californian girls with poisoning. But there was evidence of a plot in the making over several weeks, they said. They are now held at a local juvenile detention centre accused of wilful, premeditated, and deliberate attempted murder. The boys have been sent home.
In 1994, according to the FBI, 34 children under age 13 across the US were charged with murder or manslaughter. Figures for attempted murder were not available, but would presumably be larger, said Jeffrey Butts, an analyst with the National Centre for Juvenile Justice in Pennsylvania.