Rhodesian black nationalist guerrillas axed, bayonetted and clubbed eight British missionaries and four of their children to death at an isolated mission school here in the worst mass murder of church representatives and Europeans in Rhodesia's increasingly grisly civil war.

One other woman missionary was dragged into the bush and assaulted, but was still alive when she was found a short distance from the other victims yesterday morning. She was taken to a hospital in Umtali, where she was reported in serious condition.

The massacre took place Friday when a group of about 29 guerrillas visited the Emmanuel Secondary School in the heart of the mistshrouded and heavily wooded mountains 10 miles southeast of Umtali on the eastern border of Rhodesia.

The school is run by the Elim Mission of a Pentacostal church based in Cheltenham, England. The mission is active in 10 countries, including four in Africa, and operates three mission sites in Rhodesia.

The killings were the most gruesome in a series of attacks on Rhodesian mission stations that have left 33 missionaries and members of their families dead. It appears that the guerrillas' main objective is to close the schools as part of a campaign to break down government control over the rural government control over the rural population. Many missionaries believe, however, that the guerrillas are Marxists seeking primarily to undermine Christain influence in the country.

Several years ago, guerrillas killed seven Catholic missionaries at a site 35 miles northeast of Salisbury. Two Salvation Army members were killed in southwestern Rhodesia three weeks ago, and an American Baptist missionary, Archie Dunaway, recently was stabbed to death at his mission at Sanyati, 100 miles west of Salisbury.

Robert Mugabe, co-leader of the guerrilla Patriotic Front, said in a statement issued in the Mozambican capital of Maputo that "we are not responsible," and blamed Rhodesian government forces for the killings at the Pentecostal mission.

Mugabe leads the faction of the patriotic Front known as the Zimbabwe African National Union, whose guerrilla forces operate out of Mozambique only four miles from the school.

Western correspondents who went to the site of the massacre yesterday found the bloody bodies of the 12 victims still sprawled on the wet grass around the school's sports pavilion, a short distance from the main building.

There were three couples, two single women, three children and a 3-week-old baby.

The murders took place after the guerrillas had given political talks to the 200 pupils attending the school.

An ax was left in the back of a woman wearing hair curlers. Several victims had had their faces bashed in, while one man had been bayonetted repeatedly in the back.

Four of the five women appeared to have been raped, possibly when already dead or unconscious. One woman lay on her back with three children in Pajamas and bathrobes huddled around her.

One of the three men had had his hands tied behind his back before being killed, apparently by an ax.

A short twisted tree branch lay near one group of five bodies. Rhodesian soldiers at the scene said it had been used to club some of the victims.

There was no sign of a struggle.

Visibly shaken by the horror of the scene, school director Ronald Chapman said he had only Friday signed leases on homes in Umtali so that all the missionaries could live in town and commute to the school. They were to have moved next week.

Several students said the guerrillas arrived at the school shortly after 3 p.m. and ordered all of them to assemble outside the main building. Speaking in both English and Shona, the main African language in Rhodesia, six of the armed guerrillas told the students that they were to clear out no later than Monday and return to their homes.

None of the European missionary teachers were present at the meeting, although several African teachers attended and were not harmed.

"We were afraid, but I didn't understand what they were saying except that we had to begone by Monday," said one student.

It appears there were no witnesses to the killings.

One male missionary who escaped the roundup of Europeans refused to talk to reporters about what had happened.

The E1 Im mission moved its secondary school here from another site in northeastern Inyanga Park last July 27 because of the increasing danger of guerrilla attacks at Inganga. Two of the buses used in the move hit land mines and two students were killed.

The Emmanual School occupies the site of the former Eagle School, which catered to European students and was closed in 1975 because of the deteriorating security situation in the Vumba Mountains.

Now it will be closed again. As reporters were leaving the school site, the students could be seen climbing into four buses to return to their homes.

The army set out after the guerrilla band responsible for the killings, but with the Mozambique border only four miles away and the mountains covered with thick forest, there was little expectation here that they would be found.

The Rhodesian government identified the victims as Catherine Picken, 50; Elizabeth Wendy Hamilton, 37; Philip George Evans, 29; Susan Evans, 33; Rebecca Evans, 4; Peter McCann, 30; Sandra McCann, 30; Philip MoCann, 6; Joy McCann, 5; Robert John Lynn, 37; Joyce Lynn, 36, and Pamela Grace Lynn, 3 weeks.