Student protesters at the University of the District of Columbia last night rejected a tentative agreement between protest leaders and the Board of Trustees that would have ended the nine-day occupation of a campus building.

The agreement was reached yesterday afternoon. Among other things, the accord -- contained in what protest leader Mark Thompson called "a memo of understanding" -- called for an increase in the size of the board from 15 to 21 members, but it did not include a resolution of a key student demand: the resignations of the remaining six trustees.

When the agreement was put before the protesters late yesterday afternoon, a majority said they wanted to continue the occupation of Building 38, leaders said.

Afterward, a stream of students carried blankets, pillows and food into the building. "We're fortifying our position, battening down the hatches," said one student.

"We plan to stay where we are," said Thompson.

During a short afternoon negotiation, trustees told the protesters that they would agree to join in lobbying the D.C. Council to enlarge the board. But the remaining trustees, whose resignation protesters had demanded to end the occupation of the Student Affairs Building, were adamant that they would not offer to resign. Besides the six trustees named by Barry, the seventh under fire from demonstrators is UDC student Cynthia Smith, elected by the students.

Any chance of getting the remaining resignations was lost on Wednesday night, trustees said, when student protesters played for the media an audio tape from a negotiating session. The students said the voice on the tape was that of trustee Richard A. Gross, a District lawyer. "That broke the rules," one trustee said. "We agreed to let them tape-record the discussions. We did not set enough rules. But what Mark Thompson did was a terrible error."

Meanwhile, A. Knighton Stanley, who a day earlier said he would resign from the board, rescinded that resignation because of the playing of the tape, and some others said they were considering similar action.

They said the tape was taken out of context of the three-hour session.

"The trustees feel that the trust has been broken" over the tape issue, said trustee Alonza T. Evans yesterday.

Evans, an alumni representative to the board who resigned on Tuesday, said the students' action with the tape guaranteed that the trustees' position not to resign "was set in concrete."

But Thompson defended the action. "We think releasing the tape was something we had to do," he said.

On the tape played by students, Gross is heard making disparaging remarks about, among other things, the quality of instruction at the school.

"This is it," said one trustee, referring to the tape. "After this, the negotiations are over as far as we're concerned. The way they used that tape has made a difference. If they don't agree, then we move to the next phase."

Sources on the board said that use of force to remove students from the building is a last resort. But trustees ruled out bringing in police because of a possibility of lawsuits if students were injured, and the presence of some parents and children in the building.

"There were some who wanted to take a Rambo approach," a board source said of some other university officials. "But once you call in the police, you can't tell them, 'no nightsticks and no weapons.' You just don't have control."

School sources said last night that other forms of pressure may now be attempted to help end the occupation. They said that the university may seek a court order to bar the students' lawyers from Building 38, and that other actions, such as cutting phone service and limiting entry and exit at the building, may be taken to make the occupation less comfortable.

The student leaders have told the larger body of protesting students who have occupied the building since last Wednesday that they will probably be unable to force the resignations of the remaining board trustees.

Signs that the protest was beginning to wane yesterday were apparent on the Northwest Washington campus.

Crowds that had formed daily in the plaza outside the occupied building since the takeover nine days ago never formed yesterday.

The twice-a-day rallies led by student leader Thompson were scrubbed.

It appeared that classrooms were fuller yesterday than on any previous day since the strike began.

At an afternoon news conference, Thompson gave few clues that he believed his movement had wrung all the concessions it could from the university.

"From what we can see, our movement is getting stronger," Thompson said. "We need to stay in the building for as long as we can."

Thompson said compromise was not part of the student protesters' plans.

"As far as students are concerned, they've compromised enough by allowing the board to operate the way it has," he added.

Last night, some students emerged from the vote carrying signs saying "Don't sell out" and fanning out to classrooms in an apparent effort to urge night students to boycott classes.

Demonstrators have been demanding the resignation of student trustee Smith, as well as those of these Barry-appointed trustees: Gross; lawyer Charles B. Ruttenberg; educator and anti-apartheid activist Roger Wilkins; businesswoman Patricia A. Mathis; counseling specialist Concha N. Johnson; and the Rev. Ray Kemp, a Catholic priest.