South Africa's major black activist movements opted for a change of strategy today, urging students not to resume a boycott of classrooms but to attend school and try to seize control of the educational system.

Organizers of a conference here this weekend described the decision as an advance from mere opposition to the white-minority government's apartheid policies to a phase of "becoming actively engaged in formulating our own alternatives," in the words of one spokesman.

The spokeman added that the main thrust of black activism would now be work stoppages and rent and consumer boycotts, while the schools, empty throughout most of last year, would become "centers of strategic planning and organizational development."

More than 1,500 delegates from organizations of students, teachers, parents and communities, as well as all the major black labor unions, attended the conference, which was one of the most widely representative black gatherings ever held in South Africa.

Two dates for national work stoppages were named -- May 1, the 100th anniversary of the international workers' day, which was declared a "public holiday," and June 16-18, to mark the 10th anniversary of a major uprising in Johannesburg's Soweto township.

The conference also termed the Reagan administration "an accomplice in the crimes of apartheid" and called on the American people not to support its policy of "constructive engagement" with Pretoria or its aid to rebels in Angola. Another resolution called for increased western pressure on the Pretoria government through the withdrawal of investments and the denial of landing rights to the national airline.

The attempt at a display of African nationalist solidarity was diminished by the activity of black-consciousness organizations called the National Forum, which was holding a conference in Durban at the same time. The forum charged that the educational conference was dominated by the rival United Democratic Front, a much larger alliance committed to the principle of "nonracialism."

Bitter factionalism that divides black South Africans was evident in the streets even as the educational conference got under way. Members of the Inkatha movement, a Zulu group led by Chief Gatsha Buthelezi, clashed during the weekend here with rival political movements, and two persons were killed yesterday.

The conferees today formally declared the Zulu-based movement, which some whites see as a moderate alternative to African nationalism, to be an "enemy of the people" operating in league with the government.

Although Buthelezi was once a member of the African National Congress, which the United Democratic Front supports, and although he regularly calls for the release of the congress' imprisoned leader, Nelson Mandela, relations between the two sides have degenerated into often violent conflict.

The nationalists denounce Buthelezi as a "collaborator" because he heads a tribal administration within the apartheid system. It is a charge that the chief, who claims to be a more effective campaigner against apartheid than they are, strongly resents.

The decision to hold the education conference in this Indian Ocean port city was in itelf a challenge to Buthelezi, whose Zulu tribe is dominant here, and it brought the conflict to a head.

Originally the conference was to have been at Durban's Natal University, but when the office of a sociology professor who had worked for Inkatha was firebombed 10 days ago, the university authorities withdrew their invitation. Other institutions also shied away as groups of Zulus armed with sticks and spears launched several attacks on delegates as they began arriving Friday.

Yesterday, two busloads of armed Zulus -- confirmed by police to have been Inkatha members -- arrived at a community hall in an outlying white suburb that was being used as a temporary venue for registering conference delegates.

They stormed the hall, but this time some of the student militants were ready for them. They had stockpiled stones, automobile tires and gasoline plus, it seems, at least one pistol.

While terrified whites looked on from behind their high garden walls, a battle raged in the suburban street. One of the attackers was shot to death and another killed in a "necklace" execution, burned to death with a gasoline-filled tire around his body. Several more were injured, and at least six of the delegates were hospitalized.

To avoid further disruptions, the conference organizers kept secret their final venue -- a school in an Asian township several miles outside the city -- until the last moment, then continued debating through the night, finishing at 5 a.m. today.

Most of the conference was closed to the media, and the decisions were announced at a press conference this morning.

A forerunner of this meeting took place last December, when black parents, anxious about the effects a yearlong school boycott was having on a whole generation of students turned street fighters, formed a "crisis committee" to try to persuade the students to return to school for the 1986 academic year, which began in January.

The December conference produced a pact, in which the students agreed to return to school provided a list of demands put to the government were met by March 31, while the adult organizations, including labor unions, undertook to support a resumed boycott with a nationwide campaign of strikes and protest demonstrations if the demands were not met.

This conference was called to assess the situation as the deadline was reached. Although the focus was on schooling, it was seen as an occasion for the resistance movements to reassess their overall strategy.

The conference decided last night that the demands, which included a lifting of the seven-month state of emergency that President Pieter W. Botha ended three weeks ago, had been "inadequately met" and that "the situation has been exacerbated by the government's intransigent attitude."

Nonetheless, it was decided that instead of resuming the boycott, students should return to school when the new semester begins Wednesday and that they should try, with their teachers, to seize control of the educational system and change it into what is described as "people's education."

A committee was formed to define that concept more precisely, but it appears to mean the inclusion of subjects designed to awaken black self-awareness and redress what is regarded as a "white capitalist" slant on what is taught. Above all it means that blacks themselves, not their white overlords, should devise the system.

Vusi Khanyile, a spokesman for the conference committee, said it was also felt that students were better able to organize and plan campaign strategies while they were together in school than when scattered during a boycott.

In the keynote address last night, Zwelakhe Sisulu, 32, son of jailed former ANC secretary general Walter Sisulu, warned blacks that the overthrow of apartheid was not yet imminent but that a critical moment had been reached in which "decisive action can propel the struggle into a new phase."

Sisulu said the resistance movements had succeeded in their first objective of rendering many black townships "ungovernable." was being used as a temporary venue for registering conference delegates.

They stormed the hall, but this time some of the student militants were ready for them. They had stockpiled stones, automobile tires and gasoline plus, it seems, at least one pistol.

While terrified whites looked on from behind their high garden walls, a battle raged in the suburban street. One of the attackers was shot to death and another killed in a "necklace" execution, burned to death with a gasoline-filled tire around his body. Several more were injured, and at least six of the delegates were hospitalized.

To avoid further disruptions, the conference organizers kept secret their final venue -- a school in an Asian township several miles outside the city -- until the last moment, then continued debating through the night, finishing at 5 a.m. today.

Most of the conference was closed to the media, and the decisions were announced at a press conference this morning.

A forerunner of this meeting took place last December, when black parents, anxious about the effects a yearlong school boycott was having on a whole generation of students turned street fighters, formed a "crisis committee" to try to persuade the students to return to school for the 1986 academic year, which began in January.

The December conference produced a pact, in which the students agreed to return to school provided a list of demands put to the government were met by March 31, while the adult organizations, including labor unions, undertook to support a resumed boycott with a nationwide campaign of strikes and protest demonstrations if the demands were not met.

This conference was called to assess the situation as the deadline was reached. Although the focus was on schooling, it was seen as an occasion for the resistance movements to reassess their overall strategy.

The conference decided last night that the demands, which included a lifting of the seven-month state of emergency that President Pieter W. Botha ended three weeks ago, had been "inadequately met" and that "the situation has been exacerbated by the government's intransigent attitude."

Nonetheless, it was decided that instead of resuming the boycott, students should return to school when the new semester begins Wednesday and that they should try, with their teachers, to seize control of the educational system and change it into what is described as "people's education."

A committee was formed to define that concept more precisely, but it appears to mean the inclusion of subjects designed to awaken black self-awareness and redress what is regarded as a "white capitalist" slant on what is taught. Above all it means that blacks themselves, not their white overlords, should devise the system.

Vusi Khanyile, a spokesman for the conference committee, said it was also felt that students were better able to organize and plan campaign strategies while they were together in school than when scattered during a boycott.

In the keynote address last night, Zwelakhe Sisulu, 32, son of jailed former ANC secretary general Walter Sisulu, warned blacks that the overthrow of apartheid was not yet imminent but that a critical moment had been reached in which "decisive action can propel the struggle into a new phase."

Sisulu said the resistance movements had succeeded in their first objective of rendering many black townships "ungovernable."