Two Montgomery County teenagers have been charged with arson and conspiracy to commit murder in an alleged plot to kill the principal at their White Oak high school with a nail-filled bomb and then trigger a major explosion inside the school, authorities said yesterday. The Springbrook High School students -- juniors ages 18 and 17 -- are suspected of having set three fires at the school, including one Tuesday before the discovery of the plot that led to their arrests, police said. According to police, the students planned "in the near future" to throw the bomb into the principal's office,...