A Texas assistant high school principal has resigned after he was arrested Monday for allegedly having sex with an underage student in his office during the school's prom last spring.

Mark Steven West, 30, an assistant principal at Spring High School, in Spring, Texas, was charged with improper sexual relations with a student, a second-degree felony, according to the court record. The student has not been named.

West has pleaded not guilty and posted a $30,000 bond.

Charles Johnson, who is representing West, told the Harris County Court today, "Speaking with his family, looking at his background as an educator, he is innocent," reported ABC's Houston affiliate KTRK. "I think that once the truth, what he's facing and why he's facing it, comes out … the charges will ultimately be dismissed."

Image Credit: Harris County Sheriffs Office, KTRK/ABC News

West had been under investigation since last year. Spring High School Principal Donna Ulrich had told a detective in the Spring ISD Police Department that over the past school year she'd received a report from another administrator that West had made a sexual remark to a female student, and had committed a possible sexual act with a former student, according to the court record. West was also investigated by the Spring police last year for "inappropriate touching of a female student." No charges were filed because of a lack of evidence, according to the court record.

West "was instructed by administrators to discontinue all behavior that involved physical contact with female students," according to the court record.

Another staff member said she "observed female students exit [West's] office with their hair and clothing disheveled, and the female students would be smiling and laughing," the court record states.

On May 12, 2012, at the high school's 2011-12 "Project Prom" event, the unnamed student, who worked as a student aide and had been helping with the school's upcoming graduation, began to flirt with West, a witness - another student - told police. She said that "she was horny, and wanted to have sex with him," states the court record.

West protested at first, but after the girl persisted, West told her to meet him in his office, where they had sex, according to the witness.

The school district sent a letter home to parents, stating that the school "wants to assure that the safety of our students is of the highest importance at Spring High School. The criminal history and references of job applicants are routinely checked before they are hired."