South Korea's universities were warned today that no student activism would be tolerated when classes are reopened.

In a move to prevent a revival of student protests like those that preceded a military crackdown last month, the country's new education minister declared that all "collective actions" by students would be banned.

Students should be instructed to remain neutral on national issues and stay out of politics, Lee Kyu Ho, the new minister, told presidents and deans of 85 colleges and universities.

Colleges have been closed since mid-May when massive antigovernment protests swept through Seoul and several provincial cities. Two days after the students called a temporary halt to the demonstrations, a group of generals, citing the unrest as a reason, imposed rigid martial law and began arresting many dissidents and politicians.

There have been rumors that students might return to the streets when classes are reopened and no date for that has yet been set.

The new guidelines announced today are more strict than those that were in practice before the Army's seizure of power. Under a compromise reached by the former education minister, Kim Ok Gill, demonstrations were permitted so long as they remained on campus. The new rules prohibit any student action on or off campus.

Lee, who replaced Kim in a Cabinet reshuffle directed by military leaders, said that universities had failed in their obligation to create an "atmosphere of respect for law and order."

The students in early May were protesting the continuation of martial law, which had been in effect since president Park Chung Hee was assassinated last October, and were calling for the resignation of Lt. Gen. Chon Doo Hwan, the most prominent of the military leaders now running the country.

The students had not been educated to make "proper political judgements," Lee told the educators today. Campus problems cannot be settled by physical means, he said, urging the deans and presidents to open a better dialogue with the students on national political issues.

Meanwhile, an American business leader said today that businessmen are still "watching" the Korean political situation to determine the effects on private enterprise of nearly seven months of upheaval.

James M. Voss, chairman of the U.S.-Korea Economic Council, said that some American business interests may hold back on Korea investments until the political future becomes clear. He described the American attitude as one of "caution" but said he had found no vein of pessimism among U.S. members of the council.

The council has held two days of meetings in Seoul with Korean business and government leaders.

Voss, who is chairman of Cartex Petroleum Corp., said his delegation had inquired about what the military's role will be in the government from now on but had received no clear answers.

He said American businessmen were most concerned about whether the new structure of government would be as "carefully meshed" with business as before Park's death so that there would be no "undue restriction" on private enterprise. He said the council members had been assured on that point.

South Korea's economic picture was already gloomy before Park's assassination and the period of turmoil, and investors have expressed concern that one of Asia's fastest growing economies would be permanently damaged. International bankers have let it be known that they will not assent to any new long-term loans until the political scene is made clear.

A leading concern of American businessmen is that the military leaders will overrule economic planners on key decisions. The government is being directed through an elaborate structure of subcommittee dominated by generals loyal to Chon. Key economic planners who enjoy considerable respect in Western business circles have been incorporated into that apparatus of subcommittees and there has been no public indication how independent they will be from military directions.

The ruling generals have given one signal that suggests caution in interfering in business. They have let it be known that private enterprises will not be subject to a broad "social purification" campaign they have launched to clean out alleged corruption in high places.