Watson, who pleaded guilty on Thursday, now faces a sentence ranging from probation to three years in state prison, KSWB-TV, San Diego, reported.
"It's definitely a betrayal of a position of trust," Deputy District Attorney Mary-Ellen Barrett said outside court. "It's just a very sad situation."
The high school special education teacher may also be required to register as a sex offender when she's sentenced on Feb. 6.
Watson was arrested Aug. 9 on suspicion of sexually assaulting the boy, who was a student in her class at the New Haven residential school from September 2010 until August 2011, said Lt. Anastasia Smith of the San Diego Police Department.
Smith said police received a tip on July 30 about inappropriate conduct between Watson and the former student and later determined she had a dating relationship with the boy.