John Muir High School students who witnessed the fight weren't hurt.
Lt. Phlunte Riddle says 29-year-old Leta Carruthers went to the school Monday to confront the teacher. The female teacher denied the affair and there was a fight.
Officers arrested Carruthers and she was booked for investigation of making criminal threats and battery.
Pasadena Unified School District spokeswoman Binti Harvey wouldn't comment. School board president Tom Selinske called it "an action of passion," adding the adults' behavior in a classroom was inappropriate.
Information from: Star-News, http://www.newschoice.com/newspapers/losangeles/pasadena/default.asp