American companies operating in South Africa have issued a direct appeal to President Pieter W. Botha to act to "lower tensions" in his country's segregated school system as part of a program of broad reforms that could keep South Africa from facing increasing international isolation, it was disclosed here today.

Disclosure of a telex message to Botha from 186 American companies -- sent on Friday as influential businessmen from the United States and South Africa met in London to discuss a joint campaign to seek reform in South Africa -- came as violence between black mobs and police spread into new areas.

During the past two days 14 blacks were killed and at least 25 wounded in a corridor of territory between the Ciskei and Transkei "homelands" in eastern Cape Province.

The first direct move in what is expected to be a greater involvement by American companies in a joint campaign with the South African business establishment to combine pressure on Pretoria with aid projects for blacks came Friday when 186 of the 300 U.S. corporations operating in South Africa sent a joint message to Botha calling for steps to "lower tensions" in the country's black schools.

Boycotts and continuous clashes between police and student activists have caused a crisis in black education, and the message expressed concern at the effect on "thousands of young lives" and the accompanying threat of unemployment.

It is the first time any group of foreign companies has intervened so directly with the government on a domestic political issue.

The purpose of the message appeared to be to nudge Botha into ordering a relaxation of police action in black schools and universities.

Police have invaded schools and universities, arresting students and teachers. Today 15 policemen armed with shotguns were reported to have patrolled examination rooms at a mixed-race school near Cape Town, arresting some students for allegedly faking examination papers to get around an order declaring a boycott of the examinations illegal.

The 186 companies are all signatories to the Sullivan principles on fair employment practices, which were formulated by the Rev. Leon L. Sullivan of Philadelphia nine years ago.

Many also belong to a group calling itself the "U.S. Corporate Council on South Africa," which was formed in September and is pledged to play an active role in working for political reforms.

The council, comprising the chief executives of more than 50 major U.S. corporations with interests in South Africa, also organized a meeting with top South African executives in London last week to draw up a joint action program.

News of the closed meeting was published in the Financial Times of London today and has been confirmed by one of the South Africans present, Michael Rosholt, chief executive of the Barlow Rand Corp.

According to the Financial Times, the meeting was also attended by Roger B. Smith of General Motors, former U.S. Treasury secretary W. Michael Blumenthal of Burroughs Corp., the cochairmen of the corporate council, Rawleigh Warner Jr. of Mobil, John S. Reed of Citibank, and Gavin Relly, of South Africa's biggest company, the Anglo-American Corp.

Officials of the U.S. companies generally have refused to discuss the London meeting, but according to the British newspaper it decided on a strategy that includes paying for educational, housing and small business projects for blacks.

Reached by telephone from Washington, GM Chairman Smith confirmed that last week's London meeting took place. He termed the meeting "very productive."

According to officials of companies in the council, it was formed by chief executives of major companies to add a dimension of their own "personal prestige and concern" to the public debate both in the United States and in South Africa.

One official of a founding company said the executives said they "decided to . . . persuade the South African government that the issues are getting board-room and chief executive officer attention" in the United States.

On Oct. 18, the then 52 members of the council ran a full-page ad in a number of major U.S. newspapers calling for political reform in South Africa.

The Financial Times described the London meeting as part of "a major effort to present a constructive alternative to the growing move toward disinvestment from South Africa by U.S. companies."

Meanwhile, American banks were reported here today to have warned South Africa that it must move more rapidly toward political reform before they will agree to reschedule the country's $14 billion short-term debt.

The South African Press Association, the country's main news agency, quoted Wall Street sources as saying U.S. creditor banks have conveyed this warning privately to the Pretoria government.

The agency reports one unnamed broker as saying: "The banks are being very firm, but they don't want to go public with their stand, because this is essentially a political issue, and the publicity could ruin any chances of a compromise."

South Africa declared a moratorium on these debt repayments in September, when international banks, alarmed at the growing unrest and the government's apparent stalling on promised reforms, refused to roll over the short-term loans that are due for repayment at the end of the year.

Nine of the blacks who died in racial unrest yesterday and today were killed in clashes with the police in the town of Queenstown, which is in a narrow corridor of "white" territory between two tribal reservations that have been granted nominal independence under the apartheid system of racial separation.

Two other blacks also were killed in clashes in this corridor, and a 13-year-old girl was shot dead by police in the township of Zwelitsha, just across the border inside the Ciskei "homeland."

Trouble has been simmering in the corridor for several weeks. Now it has erupted into a major new unrest area, maintaining the pattern that South Africa's political crisis has followed since it began 15 months ago of dying down in one region only to flare up elsewhere.

The once-rich farming corridor has become the most depressed area in South Africa, as the country sinks into its worst economic recession in half a century.

According to sources in Queenstown, tension rose when the police conducted a house-to-house search for suspected political agitators in the town's adjoining black quarter on Friday night.

Angry blacks began rioting and the trouble escalated over the weekend, the sources said. Four blacks were killed yesterday when the police opened fire with shotguns. Another five were shot dead today.

According to an official police version of the clashes, the police opened fire because the rioters attacked them with stones and gasoline bombs. Three policemen were injured in these attacks, the official statement said.