Sen. Joseph R. Biden Jr., his Democratic presidential campaign battered by charges of plagiarism in his campaign speeches and in a law school paper, admitted yesterday that some of his boasts about his college academic achievements were untrue, including the claim that he had finished in the top half of his class in law school.

In a videotaped exchange at a campaign coffee gathering in Claremont, N.H., last April 3, Biden also claimed that he had attended law school at Syracuse University on a full academic scholarship, that he had been named the outstanding student in the political science department as an undergraduate at the University of Delaware, and that he had graduated from Delaware with three undergraduate degrees.

In a statement yesterday responding to a story in the current issue of Newsweek magazine, Biden admitted that these claims were "inaccurate."

"As the complete record of my law school career indicates, which I released to the press last week, I did not graduate in the top half of my class at law school and my recollection on this was inaccurate," Biden said. "With regard to my being the outstanding student in the political science department, my name was put up for that award by Prof. David Ingersoll, who is still at the University of Delaware."

Biden held a news conference last week at which he confirmed he had faced disciplinary action on a charge of plagiarizing part of a law school paper as a freshman, and admitted that he had sometimes borrowed quotes from other politicians, including Robert F. Kennedy, Hubert H. Humphrey and British Labor Party leader Neil Kinnock.

Biden's records that he released last week indicated he attended law school on a half-time scholarship based on financial need and that he graduated 76th out of a law school class of 85. His undergraduate academic records show that he graduated from Delaware 506th in a class of 688 with a "C" average and that he got his undergraduate degree with a dual major in history and political science.

"I did receive a scholarship based in part on academics from the Syracuse University College of Law and a grant from the Higher Education Scholarship Fund of the state of Delaware," Biden said. "In addition, the law school arranged for my first year's room and board, placing me as an assistant resident adviser in the undergraduate school." The records indicate that he had an $800 scholarship in law school against a tuition of $1,620 and that the residence adviser job paid $1,000.

He said that his reference to three degrees was intended to refer not to actual degrees but to his dual major. "I said three and I should have said two," he said in his statement yesterday.

In response to some suggestions that the Delaware bar might not have admitted him to practice in 1968 had it known of the first-year law school disciplinary action, Biden said that when he applied for admission the dean of the Syracuse Law School wrote to Delaware authorities that his "records reflect nothing whatsoever of a derogatory nature" that called into question his integrity or ability.

"Not wanting to leave any questions, however, I have written the chief justice of the Delaware Supreme Court bringing these facts to his attention," Biden said.

Biden denied other reports the last three days that suggested he had exaggerated his role in some anecdotes he has told on a high school speech he said he gave and an incident about a segregated Delaware restaurant. He also challenged suggestions he may not have won a moot court competition he said he won.

"I did, with my partner Jared Jossem, win an international moot court competition at Kingston, Ontario, on December 12, 1967," he said.

During the videotaped exchange last April that produced the latest problem for the senator, Biden appeared irritated by questions about his academic credentials.

"I think I probably have a much higher IQ than you do, I suspect," Biden responded to a questioner, according to a United Press International report on the tape. "I went to law school on a full academic scholarship, the only one in my class to have a full academic scholarship. In the first year in the law, I decided I didn't want to be in law school and ended up the bottom two-thirds of my class and then decided I wanted to stay, went back to law school and in fact ended up in the top half of my class."

His records show that he ranked 80th in a class of 100 his first year in law school, 79th of 87 at the end of his second year, and 76 of 85 the final year.

Biden took issue with a report in The Philadelphia Inquirer that questioned his claim that he had made a speech at his high school commencement "welcoming" his graduating classmates and that while in high school he and some friends left a restaurant that refused to serve a black classmate. The newspaper quoted the school's headmaster, class valedictorian and the black student as saying they had no memory of the events.

"The reports are inaccurate, and I was involved in those events," Biden said.