MERCERSBURG, Pa. - A Haitian immigrant and the daughter of an Asbury Park, N.J., taxi driver, Withney Barthelemy was not exactly the prototypical candidate for an elite prep school.

But at the behest of a teacher, she applied to a 300-acre boarding school deep in the hills of south-central Pennsylvania - Mercersburg Academy - and it offered her a full scholarship.

"It provided me with so many opportunities I wouldn't have had any other way," said Barthelemy, who moved to the United States when she was 4. "I felt like I was wanted despite the economic background I had."

She had what might be viewed as a dream high school career.

She studied in France for a month. She befriended roommates from South Korea, England, and Nigeria. She graduated in May, and is headed to exclusive Wellesley College on a scholarship for high-achieving low-income students.

Aiming to dispel the snobby prep-school stereotype, Mercersburg made financial aid the highest priority in its latest fund-raising campaign - as it turned out, one of historic proportions. On Wednesday, the school is expected to announce that it amassed more than $300 million, one of the largest fund-raising efforts ever for a prep school.

The campaign raised the school's endowment 70 percent, to $405 million, more than $910,000 per pupil. That is about 16 times the median endowment among similar boarding schools, according to the National Association of Independent Schools, among the highest per-student ratios of any prep schools and even colleges and universities.

Almost $80 million of the money raised will be earmarked for financial aid, a $3 million annual increase.

Prep schools around the country have made similar promises to boost financial aid over the years. In November 2007, Phillips Exeter Academy in New Hampshire announced that it would cover the costs for any eligible student whose annual income is $75,000 or less.

Much of philanthropist Walter H. Annenberg's $100 million gift to his alma mater, Peddie School in Hightstown, N.J., in 1993 was designated for scholarships. Before the donation, 20 percent of students received financial aid. About 40 percent of students continue to receive assistance because of the gift by the former owner of the Inquirer.

A third of Mercersburg's 441 students receive need-based financial aid to cover Mercersburg's tuition - $54,500 for boarders and $38,000 for day students - and other costs.

Douglas Hale, Mercersburg's head of school, who is retiring at the end of the month, is credited with helping shape a vision that inspired the donations during the campaign.

"The thing that is exciting about it is not just the dollar amount, but what it's going to mean for the school, the students, the faculty," said Hale, who has led Mercersburg for 19 years.

Besides financial aid, the campaign is supporting academic and residential programs and helping to build new facilities, including an alumni and parent center, a field house, and an aquatic center.

For faculty, the gift means new housing, higher contributions to 401(k) accounts, the opportunity for sabbaticals, more money for continuing education, and a new faculty training institute. Mercersburg also plans to start a teacher fellowship program for aspiring educators.

Hale said that when secondary schools raise large sums of money, "it raises the bar for everyone."

Phillips Exeter raised $352 million by the end of its campaign in 2009. In 2010, Culver Academies in Indiana announced it had raised more than $376 million over eight years. Phillips Academy Andover, in Massachusetts, raised $322 million between 2005 and 2012.

Mercersburg's "Daring to Lead" campaign to reach $300 million started three years ago, boosted in October 2013 by a $100 million gift from alumna Deborah Simon - the largest in the school's 123-year history, and one of the largest to a U.S. prep secondary school.

Simon said she was pleasantly surprised by the campaign's success.

"I didn't think we could do it," she said. "Several times last year, I thought, 'Oh, my God, we're not going to get there.' "

Nearly 6,990 donors made sure they did.

Forty-one percent were new donors. Thirty-three gifts were of $1 million and more. More than 150 gifts were of $100,000 or more. More than half the donors were alumni. One out of five donations came from parents.

Students making calls helped raise more than $300,000 during the campaign.

H.F. "Gerry" Lenfest, a 1949 alumnus and a major donor, said the endowment means a certain amount of security for the school in the future. Lenfest, who donated more than $5 million to this latest campaign, is on the board of Philadelphia Media Network, which operates the Inquirer, the Daily News, and Philly.com.

"Mercersburg is an outstanding school now," Lenfest said, "but I think with this $300 million that it will rise to the top echelon of all prep schools."

mbond@philly.com

610-313-8207@MichaelleBond