A Fairfax County grand jury added two new charges yesterday to the 10 counts police previously had filed against James Q. Stevens, the 18-year-old accused of staging a 21-hour siege at Lake Braddock Secondary School in November.

The jury indicted Stevens on a total of 10 abduction counts and two charges of using a firearm in commission of a felony. He had previously been charged with abducting nine individuals and with one firearms violation in the incident that ended Nov. 11 with his surrender to police.

The new abduction charge adds Arline Didier, 37, a reading teacher who hid under boxes in a copying room during the takeover of the school offices, to the list of hostages Stevens allegedly took during the siege. Police had not charged Stevens with abducting Didier because officers initially believed Stevens did not know that Didier was hiding in the office, a view that the teacher later said was incorrect.

Commonwealth's Attorney Robert F. Horan Jr. said the jury also indicted Stevens on a second count of using a firearm during the incident. The new count alleges that Stevens pointed a rifle at John G. Cockey, one of the first school officials to arrive at the school office after Stevens allegedly barged in firing shots into the ceiling. Cockey testified during a preliminary hearing earlier this month that he left the office under an order from Stevens to find his girlfriend among the students at the school.

Stevens, who has not yet entered pleas to the charges, is also accused of a separate firearm violation for allegedly pointing the gun at another hostage, school Principal John W. Alwood.

Stevens is being held in county jail without bond at the request of his attorneys who said the youth is undergoing counseling. He will be arraigned on the two new charges today in General District Court.