The 14-year-old allegedly posted the threat on the social-networking Internet site at the beginning of the school year, but it wasn't discovered by school officials until Monday, said Dan Ferrelli, spokesman for the Aurora Police Department.
The teen was charged with felony harassment through electronic communications, referred to the juvenile court system and released into his parents' custody, Ferrelli said. The school district is also investigating.
"It's pretty new to us," said Todd Colvin, assistant superintendent of Oswego Unit School District 308. "It's something schools are all going to have to be dealing with."
Classes at the teen's school, Bednarcik Junior High School in Oswego, ended Monday.
Last month, another suburban Chicago school district decided students would be held accountable for what they post on Internet blogs and social-networking sites. Starting next year, all students in extracurricular activities in Community High School District 128, which includes Libertyville and Vernon Hills high schools, must sign an agreement that evidence of "illegal or inappropriate" behavior posted on the Internet could be grounds for disciplinary action.