"Our school site leadership team was informed of the unexpected passing of Julius Poppinga, an 11th grade student at Westlake High School. Julius was a member of our Warrior Football and Lacrosse teams," said Principal Jason Branham in the statement.
The teen, who played defensive end for the school's football team, had been hospitalized after having breathing problems, according to the Los Angeles Times, which cited an anonymous source.
The Times said he had not played or practiced since Westlake's opener against Taft on Aug. 18.
A memorial featuring flowers, written messages and photos of Poppinga was growing outside the campus.
In an Instagram post, the school's football team said: "The Westlake Football family wants to send our deepest condolences to the Poppinga Family. Julius was a beloved member of our team and will be greatly missed by the entire tribe."
No further information was immediately available on the cause of death. This comes as a community in Lake Elsinore mourns the death of a 12-year-old student who died after P.E. class.