Live TV pictures showed swarms of police and emergency workers descending on the Marysville-Pilchuck High School in the western state of Washington.
The shooting occurred in the school cafeteria, according to multiple reports.
"I was eating... I hear four gunshots and it was behind me. I saw a gun pointed at a table... Then I ran out of the exit," one student in the cafeteria, named only as Alex, told KIRO TV news.
Television footage also showed schoolchildren coming out of the sprawling campus in Marysville, 55 kilometres north of Seattle.