Naomi Perez, 33, of Brea, also pleaded guilty to vandalizing the student's car.
Investigators say Perez and the boy had a relationship that lasted several months while she was teaching at Saddleback High School in Santa Ana in 2007.
The relationship came to light after the student reported his car had been vandalized outside his house.
Perez faces up to four years in prison.
The Associated Press contributed to this report.