The White House acknowledged yesterday that President Reagan personally gave instructions in his administration's controversial decision that originally reversed longstanding federal policy by giving tax exemptions to schools that practice racial discrimination.

The president told Treasury Secretary Donald T. Regan sometime in December that he wanted the policy changed, White House deputy press secretary Larry Speakes said yesterday. The treasury secretary then passed the president's wishes along, Speakes said.

At Tuesday's news conference, the president said, "I'm the originator of the whole thing . . . . " But it was not clear just what he had done to bring the policy change about.

The president's comment came after presidential counselor Edwin Meese III and other top officials had denied for days that Reagan had any direct involvement in the decision, depicting it as a matter that originated with Treasury and Justice department officials.

Meanwhile, officials of the American Jewish Committee said yesterday the Reagan administration is proceeding with plans to change another federal discrimination policy--the rescinding of a Carter administration rule barring federal contractors from using federal funds to pay dues to private clubs that discriminate on the basis of race, religion, national origin or sex.

Reagan's Labor Department officials placed the Carter rule under review last spring. American Jewish Committee officials said they were told this week by John Fox, legal counsel for the department's Office of Federal Contract Compliance Programs, that Labor Secretary Raymond J. Donovan would be announcing that the rule would be rescinded in a week to 10 days.

Fox did not respond to telephone inquiries yesterday. But other department officials denied that the decision to rescind the Carter rule was imminent.

"We are not talking seven to 10 days," said Dennis Minshall, special assistant to Deputy Undersecretary Robert Collyer, after conferring with his boss. "We have no definitive immediate plan to do anything." The contract compliance office recommended that the rule be rescinded, he said, but that recommendation is still officially under review.

In his comments yesterday about the tax exemption decision for private schools, Speakes refused to say just when the president gave his instruction to the treasury secretary, or precisely what he ordered done. Speakes would say only that the president "communicated his thoughts to Secretary Regan."

Reagan's original decision, which has since been modified in the face of widespread criticism from Republicans as well as Democrats, has provoked a series of conflicting explanations from the president and his aides as they tried to recover and demonstrate that the decision had not been based on racist intent.

The president made one such assertion in a telephone call last Friday to Washington Post columnist William Raspberry. Speaking of his original Jan. 8 policy announcement, the president said: "I didn't know at the time of his recent order that there was a legal case pending."

But according to a copy of a White House memo containing the president's handwritten comments, Reagan had in fact been told just that.

The memo, originally obtained by CBS News, is titled "Presidential Log of Selected House Mail." House Republican Whip Trent Lott (Miss.), it said, "writes regarding pending cases concerning the tax exempt status of church schools. Indicates that the Supreme Court has now agreed to review the case of 'Bob Jones University v. United States,' and urges you to intervene in this particular case."

To the right of that summary, in the space reserved for comments, the president had written: "I think we should." Bob Jones University was challenging the old government policy under which the Internal Revenue Service had denied it a tax exemption on the grounds that it practiced racial discrimination.

Lott was out of the city and couldn't be reached for comment. A Lott aide said yesterday that the congressman wrote the president and top officials at Treasury and Justice on Oct. 30 to urge the policy shift. In September, Justice had filed a brief in the Supreme Court supporting the long-held view that schools that discriminate aren't eligible for tax exemptions.

On Dec. 15, Lott had received a letter from Deputy Attorney General Edward C. Schmults saying the government still was prepared to defend the 12-year-old ban on exemptions to schools that discriminate.

But, sometime in mid-December, a White House aide sent Lott a copy of the president's handwritten notation that he thought the policy should be changed. So, on Dec. 21, Lott decided to help the president communicate his wishes to his officials; Lott forwarded copies of Reagan's note to Schmults and to Treasury officials, along with a second strong letter urging a reevaluation of the government position, his aide said.

Despite this, Schmults has said that by the time he received Lott's letter and the memo with Reagan's marginal note, Treasury and Justice officials already were working on the reversal of policy. But the Lott aide said wryly he was sure the note "didn't hamper their movement" toward a change in policy.

Lott had been writing letters all year to various government officials urging a change in policy because of his belief that the IRS was harassing church-related and private schools in Mississippi. For instance, last Feb. 12, Lott's office issued a press release saying he had told incoming IRS Commissioner Roscoe L. Egger Jr. just that.

Over the past several years, the IRS has lifted the exemptions of about 110 schools, including 38 in Mississippi, because of the policy against discrimination.