Michael Allen, a biology teacher at Curtis High School, made the guilty plea in Pierce County Superior Court to charges of first-degree sexual misconduct with a minor and violation of a no-contact order.
According to court documents, Allen had sex multiple times with the 17-year-old student. After he was arrested and charged, Allen continued to communicate with the victim, ignoring the court's no-contact order.
He is scheduled to be sentenced Oct. 2.
"Our schools should be safe havens," said Prosecutor Mark Lindquist. "This teacher abused the trust of his position and is being held accountable."
Court files show that Allen and the victim, who was his teacher's aide, began exchanging phone calls and text messages in January 2014.
On Jan. 17, school administrators learned the two were spending time together, and they placed Allen on administrative leave. He was told not to contact the victim.
During an internal investigation, administrators reviewed the victim's phone records and found thousands of messages she exchanged with Allen, including after Allen was placed on leave, according to the case file.
School administrators contacted the University Place Police Department on Feb. 11 to report the relationship. The victim told detectives she had sex with Allen three times after he was placed on administrative leave.
Allen was arrested and charged, and the court imposed an order prohibiting Allen from contacting the victim. Allen posted $25,000 bail and was released.
On Febr. 24, the victim told detectives that Allen had contacted her on Twitter and gave her a pre-paid cell phone so they could continue to talk. Allen was charged with violating the court's order, and a new bail was set in the amount of $125,000.
He has been held in custody since March 20.