Former high school senior Stephen L. Bonner was indicted yesterday by a Montgomery County grand jury on four felony charges stemming from the late February ransacking of Richard Montgomery High School in Rockville.

Bonner, who turns 18 on Wednesday, was one of two suspects charged with breaking into the school twice during the weekend of Feb. 23-25.

They are accused of burning office files and library books, smashing computers and other expensive electronic equipment, breaking windows and trying to cause an explosion by turning on five natural gas valves in a chemistry laboratory.

Authorities have described the damage, estimated at $250,000, as the most destructive act of vandalism ever to occur at a Montgomery County school.

Bonner initially was charged as a juvenile, although county prosecutors indicated at the time they planned to push to have him charged as an adult. The second suspect, Jason W. Knight, 18, of Rockville, was charged as adult in March with 10 felonies. Knight is being held in the county Detention Center in lieu of $100,000 bond. Bonner remains in the custody of his parents in Rockville.

Last month, Montgomery County District Court Judge Douglas H. Moore decided to move Bonner's case out of juvenile court and allow the youth to be prosecuted as an adult, said Bonner's attorney, Jeffrey T. Wennar.

Juvenile court proceedings are closed to the public, but Wennar said he argued unsuccessfully at the three-hour hearing to retain juvenile status for Bonner, citing the youth's age and lack of a prior criminal record.

Bonner was indicted yesterday on one charge of breaking into the high school with intent to commit a felony, two counts of destruction of property and one count of attempted arson. Bonner could be sentenced to 26 years in prison if convicted on all the charges.

However, Bonner, who has been cooperating with authorities, was not indicted yesterday on the most serious charge, of attempting to set off an explosion.

Knight was charged with that offense, which is punishable by life in prison.

Police said Bonner and Knight illegally entered the high school about 11 p.m. Feb. 23, and during a nine-hour rampage smashed typewriters and computers and tried to set fire to an office after pouring charcoal lighter fluid in the room.

Authorities said the pair returned Feb. 25 to the school for six hours and demolished classrooms, electronic equipment and library books.