A substitute teacher at an alternative school in Auburn was indicted this week on a charge that he had sexual relations with a teenage student last year. Tyler Gadaire, 31, of Turner, was charged with a single count of gross sexual assault. He is accused of having an ongoing relationship with the student that involved sexual contact in a classroom, secret meetings in a school quiet room and encounters at a Bath hotel. Gadaire is accused of having sexual relations with the girl who was then 17 and a student at the Androscoggin Learning and Transition Center, a behavioral alternative school on Poland Road. While investigating the alleged sexual relationship, Auburn police worked directly with investigators from Bath.