Stretton said afterward that Williams had no previous criminal record and that before he was named dean of students at Northeast, he was an assistant principal at Overbrook High School and a police officer in California.
Judge Diana L. Anhalt allowed Williams to remain free on $50,000 bail, pending sentencing on Sept. 12.
Assistant District Attorney Kelly Harrell said she would ask for prison time. The serious charge, unlawful contact, could carry a term of three to 12 months, based on sentencing guidelines, she said.
"He was someone she should have been able to look up to," Harrell said, "and he took advantage of her."
Harrell said no DNA evidence incriminated Williams. The verdict showed the jury believed the student's testimony, the prosecutor said.
Williams was arrested two months after he took the girl out for a meal on the Martin Luther King holiday and then stopped by his Exton home, where he was accused of having sex with her.
In testimony, Williams confirmed the prosecutor's account except for denying having sex with the girl.
Harrell said the girl's sister overheard a phone conversation between her sister and Williams and told her mother, who reported Williams to police.
Harrell said the girl was in custody on violation of a juvenile criminal charge: "She is very troubled and this incident didn't help."