School Principal Michael Vossen sent an email to players' parents notifying them on Saturday.
Coach Larry Hirst said the threats were of a physical nature. Hirst, 50, took a leave of absence as coach in January amid a 10-game losing streak.
Vossen said possible suspects include individuals associated with the school's basketball program. Police are investigating.
Mark Okey, whose son is on the team, told the Orange County Register the threats were in the form of letters left at Hirst's home in Huntington Beach. Another was left on a car driven by Hirst's wife, Sheridan, who is a teacher at Newport Harbor.
Okey said six players, including his son, were pulled out of class on March 13 and questioned by campus police.
City News Service contributed to this report.