During his junior year, Glennon threw for over 1,700 yards and 21 touchdowns while rushing for another four touchdowns. Over the next year, his final (senior) season at Westfield High School, Glennon led the Bulldogs to the 2003 Virginia AAA Division 6 state championship. He passed for 1,840 yards, 26 touchdowns, and four interceptions during the championship campaign and added four touchdowns on the ground. Following the championship season, Glennon received numerous accolades as he finalized his search for a college choice. In total, Glennon amassed over 4,800 yards passing, 67 passing touchdowns (7th all-time in Virginia), eight rushing touchdowns, and only 11 interceptions.