Police shot one black youth dead and seriously wounded another in Soweto today after raiding a high school where, they said, pupils were holding lessons on how to make fire bombs and booby traps.

Eighteen teachers and 109 students were arrested at the Moletsane High School in the tense black township outside Johannesburg, police said. This brought the number of arrests in raids on Soweto schools this week to more than 340.

The shooting took place as thousands of Soweto high-school students continued a three-week-old class boycott, despite student leaders' urgings that they return to classes and official threats to close or take over most of the Soweto high schools.

The South African government has issued an ultimatum that 40 schools will be shut down unless pupils return to their classrooms by next Thursday.

T.S. Kambule, principal of Orlando High School in Soweto, said earlier this week that if the black schools are closed, the situation would become "very explosive" because "The black children of today are no longer passive like we of the older generation are."

Police said Desmond Mabuse, 19, was shot after police raided Moletsane school and reportedly found about a hundred youths throwing rocks at vehicles. A police spokesman said five shots were fired from a service revolver to disperse the crowd.

Officials said that they had received a tip about the bomb-making class and that blackboard diagrams found in classrooms confirmed the tip.

The black Johannesburg newspaper, The World, quoted an unnamed student witness as saying that the shooting came after the raid, when police turned on a crowd that had gathered near the school. The witness denied any stonings or bomb-making lessons.

Mabuse is the fifth black to be shot dead by riot police in Soweto since the boycotts begn three weeks ago to back demands that education for blacks be made equal with that for whites.

Soweto student and community leaders blamed the continuing boycott on police raids, which they claim are a deliberate attempt to keep students out of school.

Dr. Nthato Motlana, chairman of a group of Soweto community leaders, said he thinks police are trying to keep pupils out of school to destroy the "power base" of the militant Soweto Students' Representative Council. That group has urged students to return to school.

Police Deputy Commission Maj. Gen. Dawid Kriel described the claim as an "infamous lie."

Meanwhile, the government banned all unauthorized meetings of more than five persons in Bophuthatswana, a black independent Dec. 6. No reason was given for the ban, but general elections are scheduled in the homeland next week.