A former high school teacher in Fairfield has been charged with sexual assault after an investigation determined that he had a sexual relationship with an underage female student, police said Thursday. Glenn Mishuck, 47, of Bridgeport was charged with 17 counts of second-degree sexual assault. Mishuck, who taught at Fairfield Ludlowe High School, was being held on $150,000 bond. There was no immediate response to a message left with his attorney Thursday. Detectives received information Sept. 18 that there might be an inappropriate relationship between a teacher and student at the high school, police said.