A second youth convicted in what a judge called "unbridled destruction" last February at a Montgomery County high school was sentenced yesterday to 12 months in jail.

Stephen L. Bonner, 18, pleaded guilty in October to malicious destruction and attempted arson over a two-day period at Richard Montgomery High School in Rockville. The cost of the vandalism is estimated at $650,000.

Bonner and co-defendant Jason W. Knight illegally entered the school on Feb. 24 and set numerous fires in the building, opened gas jets in science labs and smashed windows and equipment. The following night, the two returned to the school, demolishing a television studio and burning and destroying the school's media center and library card catalogues.

Bonner's sentence was six months less than that given Monday to Knight, who had pleaded guilty to the same charges. Knight, 19, was ordered to serve 18 months in the Montgomery County jail.

Circuit Court Judge DeLawrence Beard, who sentenced both teenagers, said he gave Bonner a shorter sentence because he cooperated with police and prosecutors.

But Beard, who denied a defense request to stay Bonner's sentence until after Christmas, said the extensive damage to the school justified time behind bars. "The bottom line is that you and Mr. Knight perpetrated this offense together. It is appropriate that you go to jail together."

Bonner apologized to Beard and a courtroom filled with Richard Montgomery students for his actions. Bonner, who was a senior at the school when he vandalized it, said he and Knight "acted out of frustration and anger without thinking of the consequences."

Defense attorney Jeffrey Wennar asked that Bonner, who was released to his parents' custody after the incident, be ordered to perform community service and deliver speeches to county students on his case. Prosecutor Constance Junghans testified for the defense, saying that while Bonner should be given credit for his cooperation, he also should be punished.

After the sentencing, David Zipp, a junior at Richard Montgomery, said the memories of the devastation remain strong. "When I walk by the library, I can still feel and remember the black construction paper that was once there. I remember the smell of smoke."