Two Iowa teenagers charged in the beating death of a high school teacher have pleaded guilty to first degree murder.

Willard Miller and Jeremy Goodale both changed their pleas to guilty on Tuesday and admitted helping to kill 66-year-old Nohema Graber, their Spanish teacher at Fairfield High School.

Investigators said the attack was possibly regarding a dispute over grades in Ms Graber’s class.

Prosecutors say the evidence shows both Miller and Goodale struck Ms Graber with a bat. Miller admitted helping plan and carry out the killing but has denied ever hitting Ms Graber.

The teacher’s body was found in a Fairfield park in November 2021 hidden under a tarpaulin, wheelbarrow and railroad ties. The town of Fairfield is home to some 9,400 people and is around 100 miles (160 kilometres) south east of Des Moines.

Prosecutors agreed to recommend a sentence between 30 years and life in prison with the possibility of parole as part of an agreement with Miller. Prosecutors said they would recommend a sentence between 25 years and life with the possibility of parole for Goodale.

Goodale previously agreed to testify against Miller at his trial, which was expected to begin later this week.

Goodale and Miller were both charged as adults because of the severity of the crime, but they were not subject to a mandatory sentence of life without parole because they were 16 at the time Ms Graber was killed. Willard is now 17 and Goodale is 18.

Investigators have said that Miller met with Ms Graber at Fairfield High School on the afternoon of November 2, 2021, to discuss his poor grade in her class.

Ms Graber later drove her van to a park where she was known to take daily walks after school, authorities say. Witnesses saw her van leaving the park less than an hour later with two males in the front seat.

The van was left at the end of a rural road. After getting a phone call from Goodale, a witness later picked up Goodale and Miller as they walked to town on that road, investigators say.

In a police interview, Miller described the frustrations he had with the way Ms Graber taught Spanish and over how the grade in her class was lowering his grade point average.