The second of two former Montgomery County high school students charged in one of the county's worst cases of school vandalism pleaded guilty yesterday, after telling police that damaging a Rockville high school "seemed exciting."

Stephen L. Bonner, 18, pleaded guilty yesterday to two counts of malicious destruction of property and one count each of breaking and entering and attempted arson.

Bonner faces a maximum sentence of 26 years in prison at his Dec. 13 sentencing before Circuit Court Judge DeLawrence Beard.

Bonner and Jason W. Knight, a friend and former classmate, were charged in March with illegally entering Richard Montgomery High School twice during the Feb. 23 weekend and causing about $650,000 in damage.

Knight, 19, pleaded guilty Oct. 1 to the same four charges. He is scheduled to be sentenced before Beard Dec. 3.

Yesterday, Assistant State's Attorney Mark Hall said Bonner told police he had participated in the rampage because "it was something to do."

Hall said Bonner, who was 17 and a senior at Richard Montgomery at the time of the vandalism, also told authorities he was angry about attendance problems he had at the school.

Bonner, who has cooperated with police in the investigation, initially was charged as a juvenile because of his age. However, he was indicted as an adult in June by a Montgomery County grand jury on felony charges.

About a dozen Richard Montgomery High School students, accompanied by Principal Thomas Quelet and several teachers, watched yesterday's hearing. Quelet said the students were chosen by lottery because of school interest in the case.

Bonner, dressed in a gray suit, spoke sparingly at the hearing. Responding to questions from Beard, Bonner said he had completed a high school equivalency course and was receiving counseling from a psychiatrist.

In a lengthy statement yesterday, Hall detailed the destruction of the school during the late night hours of Feb. 24 and 25. On the first night, Hall said Bonner turned on natural gas valves in the chemistry labs, smashed windows, dumped lighter fluid on floors and scrawled graffiti on walls.

Hall said Bonner and Knight returned the next day to the school and burned books in the library, demolished an instructional television studio and set several small fires.