ATLANTA -- The number of murders committed by students in U.S. schools or at school-sponsored outings rises when students return to the classroom at the end of summer and Christmas holidays, Reuters quoted federal health experts as saying on Thursday.

A study by the Centers for Disease Control and Prevention revealed that school-related homicides typically were highest at the beginning of the academic year in September and at the start of the spring semester in February.

CDC researchers said that the spike in the school-related murder rate might be linked to the high stress that students were typically under when they returned to classrooms or to unresolved conflicts that festered during holiday breaks.

The study was released as millions of U.S. children were preparing to return to school following the end of summer holidays.

"It takes awhile, particularly at the start of the year, for students and teachers to develop a routine and become comfortable with scheduling, class composition and things like that," said Dr. Mark Anderson of the CDC's National Center for Injury Prevention and Control.

The CDC study used as a starting point 209 media reports of violent school-related deaths in elementary and high schools between 1992 and 1997. Researchers called schools and other officials to confirm information and then did their own statistical analysis of the data.

The study also showed that students were most likely to kill themselves during the spring, a finding consistent with trends among the general population.

School-related homicides occurred at a rate of one for every seven school days, while school-related suicides occurred at a rate of one for every 31 school days, according to the study. Most school-related deaths occur in high schools.

School violence has become a hot issue in the United States in the wake of a rash of fatal school shootings, including a 1999 massacre that left 14 students and one teacher dead at Columbine High School in Littleton, Colorado.

Last month, a 14-year-old Florida boy was sentenced to 28 years in prison for fatally shooting a teacher in May 2000.

Responding to growing concerns among parents and educators, the FBI last year released a checklist of 40 warning signs that might help school officials assess possible threats of student violence.