Robert Schanne, 43, said his affair with Jenna Addis didn't start until the year after her 2003 graduation from Lower Merion High School in suburban Philadelphia.
The lawsuit, filed Wednesday, describes their relationship as "an occasional intimate relationship from 2004 to 2010." Addis, 26, grew jealous after he told her last year he was seeing someone else, and threatened to "manufacture details about an alleged improper relationship," the lawsuit said.
She told Lower Merion officials in December that they had had a sexual or romantic relationship while she was an 18-year-old student, the lawsuit charged.
Schanne was fired the following month. He had earned about $100,000 a year after more than 13 years in the affluent district.
"I think his teaching license is in jeopardy," lawyer Timothy Kolman told The Associated Press. "He's an excellent teacher, and I know that many of the students and many of the parents are distressed that they no longer have his teaching talents to rely on."
A phone number for Addis in New Orleans could not be determined. School district spokesman Doug Young said officials had no comment.
According to the lawsuit, Schanne taught Addis in both her junior and senior years. She often ate lunch in or near his classroom, and he tutored her at her home, it said. Addis allegedly told school officials the tutoring sessions included times when they were alone at her home, and that their relationship came to cross the line, through hugging, kissing and other physical contact.
According to Kolman, Schanne was fired not for any lapse in judgment for the home tutoring sessions, but for the allegation that he and Addis engaged in an illicit affair.
The lawsuit seeks more than $75,000 in damages. Schanne may also ponder a wrongful firing lawsuit, Kolman said.