Montgomery County school authorities have expelled from Western Junior High School the 15-year-old son of a high-ranking diplomat at the Embassy of Gabon whom police and fire officials identified as the chief suspect in two fires that were set at the school last month.

The youth was suspended April 28. His expulsion will ast at least for the rest of this school term but can be appealed to the school superintendant.

County authorities have said they are unable to arrest the youth and charge him with two counts of arson and one of burglary because, as the son of an embassy official, he has diplomatic immunity form criminal prosecution.

Both the youth and his mother have denied the charges in separate phone converstions with a reporter.

State Department officials have met twice with representatives of the Embassy of Gabon to present the evidence gathered in the case and to discuss the possibility of the embassy stripping him of his diplomatic immunity. A third meeting is scheduled for next week.

School authorities have made no decision about admitting the youth in the fall.

The two fires occurred a few days apart in late April and caused about $31,000 in damage. The first fire started in a physical education teacher's office, according to authorities, the day after the youth had argued with the teacher for confiscating a tape recorder and locking it in his office. The tape recorder was missing after the fire, investigators said.