President Reagan invited a 17-year-old Presidential Scholar into his office yesterday to receive her petition for a nuclear-weapons freeze and to discuss defense and disarmament.

After their 20 minute private talk, Ariela Gross, of Princeton, N.J., said the president was "charming," but she added that it was "terrifying to hear in person that he so strongly believes" that building American military strength is the way to arms reduction.

At an awards ceremony an hour later on the White House lawn, however, the president received warm applause from the scholars, their parents, and teachers, when he said they had an obligation "to ensure that America is strong enough . . . to remain both free and at peace."

Gross told reporters that her petition was signed by 14 of the 141 scholars, who were picked by a presidential commission as the top high school seniors in the nation.

At the start of the group's week-long visit to Washington, she said 24 of them had sent her letters saying they would sign her petition, after she mailed them copies of the statement.

Gross, the daughter of a Princeton University physics professor, said she received "intimidating" phone calls from Gary Stember, executive director of the scholarship program, warning that her $1,000 scholarship might be "rescinded" and the White House ceremony canceled unless she dropped the petition.

Gross said she phoned the Associated Press about Stember's statements and a few hours later received a call from an executive assistant to Education Secretary Terrel H. Bell, telling her that Stember's statements were not authorized.

Bell, who said Stember's call was "mistaken," told reporters that any threats were "abhorrent" and said Gross "has a right to do what she wants to do." Stember has declined to comment.

The petition drive stirred a sharp debate among the scholarship winners, and yesterday several expressed dissatisfaction that Gross received so much attention.

"It's a shame that she got 20 minutes with the president and we got less than ten and not even individually," said Kevin Berlin, 18, of Potomac.

"I guess Reagan felt obligated to do something . . . It's wonderful for her, but she wasn't representative of the group. We're not a group of political activists."

"I didn't come here for a freeze rally," said Mark A. Morris, 18, of Des Moines, who said he originally planned to sign the petition but changed his mind though he still supports a freeze on U.S. and Soviet nuclear weapons.

"It's too bad all this is pulling attention away from education, which is why we're here," he added.

In her petition, Gross said the scholars were "gratified by your confidence that we will play a part in shaping the future of this country. We cannot, however, accept this high honor without voicing our most profound fears for the future itself."

"I pray for the day when nuclear weapons will no longer exist anywhere on earth," Reagan said at the ceremony. But he added: "We're keeping our military strong for only one reason--to deter any adversary from thinking it can achieve its goals through war."