Trustees at the University of the District of Columbia failed to reach an agreement yesterday with students occupying a campus building, but they vowed to reopen the school on Monday, setting the stage for a possible confrontation with the students over the weekend.

The trustees said last night they were considering imposing a deadline on the students to vacate the building. But they would not say whether force would be used to remove them. Classes, which were canceled yesterday and Thursday because of the protest, were canceled for the weekend as well.

"This board of trustees is prepared to open this institution first thing Monday morning," Trustee A. Knighton Stanley said at a news conference on the Van Ness campus last night. "We would hope that we could open school the way we usually do."

After he spoke, students occupying Building 38 raised anew the chant of "the board must go."

Mark Thompson, a leader of the student protest, said the students would occupy the building "until further notice."

A group of student protesters met with trustees at the campus for about 15 minutes yesterday afternoon. No progress was made on any of the student demands, according to people who attended the session. "They came in, we thought, to talk about concerns, but all they wanted to hear was about our resignations," one trustee said.

Late last night, the students and the board resumed negotiations on several other student demands: total amnesty for protesters, longer library and cafeteria hours, day care on campus, development of an Afrocentric curriculum and the renaming of some campus buildings for noted black Americans.

The trustees sent responses back to the students and were awaiting a reply. The board said it would resume negotiations at 10 a.m. today.

It was unclear whether the students were changing their negotiating position, which has centered on the resignation of the trustees, or were expanding the scope of earlier negotiations.

The several hundred students occupying the building have maintained they will not leave until the 11 trustees appointed by Mayor Marion Barry resign. They also have called for the resignation of the trustee chosen by the students, but later said they were rethinking that demand. The remaining trustees on the 15-member board are appointed by alumni groups, and have not been the target of student anger.

The students contend that the trustees have been uncommunicative and insensitive to student concerns about their management of the university.

The trustees left the university shortly after 7 o'clock last night, but not before discussing with campus police measures they could take to prevent supplies and more students from entering the building, which the students have occupied since shortly before noon Wednesday.

The students originally occupied two buildings near the Connecticut Avenue entrance to the university, but relinquished control of Building 39 yesterday so they could meet with the board in its administrative offices.

There were several developments yesterday on the board's side. Nira Hardon Long, the board chairwoman, said she would resign the chairmanship effective Oct. 16, the date of the board's next regular meeting, in an effort to end the takeover.

"The board urged me not to do that, but I'm doing it for the good of school," Long said. "That's the most important thing." Other board members said the effort to retain Long was not unanimous.

Long said last night that she doesn't understand why the students have focused on her role as an issue.

"The students don't even know me well enough to pronounce my name," she said.

Trustee Arthur M. Reynolds, one of the mayoral appointees, announced his resignation yesterday afternoon while the board was still waiting to meet with the students, according to sources familiar with the negotiations. He left the university without commenting.

Lourdes Miranda, another of the mayoral appointees, had resigned from the board about three weeks ago, the sources said. Her decision has not been disclosed by the board.

The immediate resignations of Long and Cynthia Smith, the student representative to the board, were two key demands of the students, but the students later extended their position to include the resignations of the other 10 trustees appointed by Barry.

Smith refused to resign. The other mayoral appointees, in an early morning meeting with Barry yesterday, rejected the student demand, saying mass resignations would cause more chaos at UDC. Barry later met with student protest leaders for 2 1/2 hours, but no progress was made.

"It seems to me they ought to sit down face-to-face and discuss what the issues are," Barry said after a late morning meeting with student leaders.

The mayor's tone had changed from Thursday when, after meeting for more than four hours with students, he said he would urge Long and Smith to resign immediately.

Yesterday, Barry said his function in the negotiations was that of messenger.

For most of Wednesday and Thursday, students saw Barry as a key supporter in their efforts to reshape the board. The masses of students enthusiastically applauded whenever protest leaders mentioned the mayor's name. But after Barry returned to the Van Ness campus yesterday without the resignations of his trustee appointees, the students began to view Barry in a different light.

"When he talked to us after {Thursday's} negotiations, if we'd have had our eyes closed, we would have thought it was the board of trustees talking to us," said one fourth-year student.

About 5:30 p.m. yesterday, Barry returned to the Van Ness campus in an effort to move along discussions between a group of about 20 student protesters and the Board of Trustees. But again, there was little progress.

"We kept asking about the resignations and they kept saying they didn't understand the question," one student said.

As he left the university, Barry described the situation as "beyond impasse."

Last night, Barry said, "I'll go back whenever I think I'm needed.

"I support the overall direction of things," he said of the students' general demands for campus improvements, "but I did not like the position they were going to have: a non-negotating posture influenced by their lawyers."

Many city officials and political leaders have met with students since the protest began. The latest to arrive last night was Jesse L. Jackson, who visited the students in Building 38 about 10 p.m.

Although the resignations of the trustees have been the focus of the students, they have a list of more than 20 other demands, including the abandonment of board plans to spend $1.6 million to renovate UDC's Carnegie Library to house an artwork called "The Dinner Party." The students want longer hours at the study library, a curriculum that gives more consideration to African perspectives and a task force to revise athletic programs.

The 12,000-student university, created 13 years ago, has never had administrative peace or a generally accepted set of goals, but this is the first time that student anger over those problems has reached such a dramatic level.

This year, the school has been plagued by continuing problems in the management of its athletic programs, the firing the university president, Rafael Cortada, and the controversy over the board's plan to acquire and exhibit "The Dinner Party."

Students said their anger with the board increased last Monday when the board refused to meet with them until its regularly scheduled session on Oct. 16. They blame Long for refusing to call a special meeting and accuse Smith of failing to keep them informed of the board's actions.