Students who had been evacuated told reporters the shooting occurred in the cafeteria of Arapahoe High School in suburban Denver on Friday afternoon.
The student who was shot was in critical condition, The Associated Press reported.
Law enforcement officials told reporters the gunman had been found dead inside, possibly from a self-inflicted gunshot wound.
He was a student at the high school, and had intended to target a specific teacher, who escaped unharmed, Arapahoe County Sheriff Grayson Robinson said.
Arapahoe High School is not far from Columbine High School, the scene of a 1999 massacre in which two students murdered 12 students, a teacher and wounded several others before the pair committed suicide.
The shooting also comes one day before the one-year anniversary of last year's shooting at Sandy Hook Elementary School in Newtown, Conn.
Gunman Adam Lanza gunned down 20 first-graders and six staff members before killing himself.