Thirty-nine-year-old Gabrielle Marvelli pleaded guilty Tuesday to a single charge of corruption of a minor in Lehigh County Court.
The former Allen High School teacher admitted having sex with the student, who was a week away from his 18th birthday at the time of the illicit contact.
The boy's mother told WFMZ-TV back in June that her son later regretted his decision to have sex with Marvelli and decided to come forward and tell his mom about it.
Authorities say Marvelli and the student had an early morning encounter on May 28, the same day as the school's senior prom.
The Allentown School District says Marvelli is no longer a teacher there.
Marvelli and her attorney declined comment after the hearing.
Sentencing is scheduled for December 20.