HONOLULU -- A former teacher's aide at McKinley High School on Monday morning pleaded guilty to sexually assaulting a special education student this past May on campus.

Prosecutors and police said Gregory Keau, 30, forced a 17-year-old special education student to have sex with him.

"Another employee of McKinley High School walked in on the victim performing fellatio on the defendant," Deputy Prosecutor Thalia Murphy said.

The girl told police that Keau also sexually assaulted her in the same classroom the year before.

Keau on Monday told the judge he spoke with his attorney and changed his plea from not guilty to guilty to two felony and two misdemeanor sex assault charges. Keau has to register as a sex offender.

The prosecution said the plea will spare the victim and her family the pain of having to go through a trial.

He faces a maximum of 10 years in prison on each of the felony sex assault counts.

"These could be stacked up to 20 years prison. You understand that?" Judge Michael Town said.

"It was brought to my attention. Yes," Keau said.

The misdemeanor sexual assault counts each have a maximum of a year behind bars.

"I would also like to add that the state were to move for consecutive sentencing in this case, the defendant would be eligible for 22 years," Murphy said.

That is a move the state plans to make.

Keau said nothing further besides answering the judge's questions.

Sentencing is scheduled for Oct. 1.