OKLAHOMA CITY (REUTERS) - Public schools in Duncan, Oklahoma, were on alert on Wednesday after police received threats against Duncan High School following the arrest of three teenagers in connection with the killing of a university student from Australia.

The school, in a town of 24,000 about 130km south of Oklahoma City, was told of the anonymous threats on Tuesday night by police, schools superintendent Sherry Labyer said in a message on the Duncan Public Schools' website. The nature of the threats was not known.

Christopher Lane, 23, of Melbourne, was killed by a gunshot in the back on Friday as he was jogging in Duncan.

Duncan public schools were requiring students to stay on campus until a parent or guardian checked them out, Labyer said.

She said children whose parents preferred to keep them home on Wednesday would be excused, Ms Labyer said.

"Please know that we are committed to taking all reasonable action to protect the well-being of our students," said Ms Labyer.

A representative for Duncan High School declined to say if the suspects attended that school, and referred questions to police, who did not immediately return calls for comment.

Chancey Allen Luna, 16, and James Francis Edwards Jr., 15, have been charged with first degree murder, according to the Stephens County District Attorney's office. Michael Dewayne Jones, 17, has been charged with use of a vehicle in the discharge of a weapon and accessory to murder after the fact, the District Attorney's office said.

All three have been charged as adults, though Jones is considered a youthful offender.

District Attorney Jason Hicks said he intended to pursue adult sentences for all three. In Jones' case that would be 5 years to 45 years for being an accessory after the fact, and 2 years to life for use of a vehicle in the discharge of a weapon.

"I will be shocked if he is not found to be eligible to be sentenced as an adult," Mr Hicks said of Jones.

Terrell Cox, 16, a student at Duncan High School, told Reuters the three suspects were a "stand-off group that didn't seem to care that much."

Cox said the atmosphere "pretty hectic" at his school on Wednesday. "People are either very scared or just not bothered,"Cox said. "This is not normal for Duncan... It's like culture shock."

Lane was jogging during a visit with his girlfriend and her family in Duncan when he was shot, police said. He attended East Central University in Ada, Oklahoma, where he was on a baseball scholarship.

According to police, Jones said the teens decided to kill someone "for the fun of it." Police said Jones told them on Sunday that the three teenagers saw Lane jogging and decided he would be their target, followed him in their vehicle and then shot him in the back.

If convicted, Luna and Edwards face a possible sentence of life in prison without parole, according to court documents.

Under a 2005 United States (US) Supreme Court ruling, they could not receive the death penalty because of their ages.

The next court hearing for Jones has been scheduled fore October 2, and a preliminary hearing for Edwards and Luna has been set for October 16.