The incident happened Thursday.
School leaders got a tip that a student had a loaded gun.
SEE ALSO: Student in custody after bringing gun to Highland Park school: police
Officers arrested the Willowbrook teen at the school, and took him to the police department for questioning.
He faces weapons charges.
He's being held at least until his next court appearance, which is Dec. 2.
"There is absolutely no reason whatsoever for a sixteen-year-old boy to be in possession of a loaded firearm, particularly at a school, as alleged in this case," DuPage County State's Attorney Robert Berlin said in a statement Friday.