A lawsuit filed in Brooklyn's federal court this week claims teachers' union president, Michael Mulgrew, was caught having sex with a staff member at a Brooklyn High School.

The lawsuit filed by a Manhattan math teacher states that a custodian found Mulgrew having sex with guidance counselor Emma Camacho-Mendez in a wood shop classroom at William Grady High School.

The alleged incident occurred before 2008, when Mulgrew taught at the high school.

Andrew Ostrowsky, an instructor at Frank Sinatra High School of the Arts in Manhattan, filed the 73-page lawsuit, which states the United Federation of Teachers conspired with education officials to cover up the scandal.

Schools Chancellor Dennis Walcott, Mulgrew, the teachers union and Mayor Michael Bloomberg are all named in the suit. The lawsuit claims the city used the incident to blackmail the union into making labor concessions.