Somerset County Sheriff's Deputies arrested Duane Marquis, 58, of Skowhegan on Thursday.
Administrators at Carrabec High School notified the Sheriff's Office about the alleged relationship between Marquis and an 18-year-old student in the driver's ed program.
Police say Marquis admitted to the relationship in an interview with police and was charged with gross sexual assault.
According to Sheriff Dale Lancaster, Maine law states that if the victim is a student enrolled in a public or private school or facility, and the actor is a teacher or person with supervisory or disciplinary authority over the student, then a gross sexual assault has occurred.
Marquis was taken to the Somerset County Jail, where his bail was set at $2,500 cash.
Marquis is also prohibited from contacting the victim or the victim's family.
His first court appearance will be May 15.