Larry Hirst, 50, who coached the team for more than 15 years, stepped down as coach after receiving the threats.
"He was very fearful for himself and for the safety of his family and ended up quitting as a result. So that's a very serious situation," said Newport Harbor High School principal Michael Vossen.
Vossen won't say exactly what threats were made but confirms something was left at the school, where Hirst's wife also works. There was also some vandalism at their home in Huntington Beach last week.
"I heard someone put sticky notes all over his wife's car, just saying 'Leave Harbor or bad things will happen,'" said student Brandon Beck. "They just hate the coach, they don't want the coach to be part of the program."
According to Vossen, police suspect those behind the threats are persons associated with the basketball program, which has about 30 to 40 students.
Police won't talk about specifics but confirm an investigation is under way. In the meantime, the boys basketball program is suspended. Some students think the controversy hurts the entire school.
"The whole team is suspended, that's just unfair and whoever threatened our coach needs to come out and give a public, official apology not only to the coach but all of our student body," said student Donovan Acadine.
Hirst took a leave of absence as coach in January in the midst of a 10-game losing streak. He returned two months later until he began receiving the threats.
"Any individuals found to have participated in this they'll be prosecuted to the fullest extent of the law both from the criminal aspect as well as from the school side," Vossen said.