CHARLESTON, W.Va. -- A former instructor at George Washington High School pleaded guilty to sexual abuse Thursday.Martin Dale Kirby, 44, of Dunbar, entered a Kennedy plea to the charge of sexual abuse by a parent, guardian, custodian or person in position of trust, according to assistant Kanawha County prosecutor Michele Drummond.A Kennedy plea is like a guilty plea in most respects, except that the defendant does not actually admit guilt.Kirby was an instructor for the Army Junior Reserve Officer Training Corps at George Washington High.Prosecutors say he had sexual contact with an underage female student more than once in September 2010 at the school.Kirby admitted to having sex with the student, according to police. He faces 10 to 20 years in prison when Kanawha Circuit Judge Jennifer Bailey sentences him March 11, Drummond said. Kirby served as a sergeant major in the U.S. Army before joining the school as an instructor. He had taught eight JROTC and learning skills classes, according to the school's website. The victim had been a student in his class.