Destiny Miller went online this summer to check one last set of grades from her senior year at Woodlawn High School — scores on three Advanced Placement exams.

The 18-year-old sat alone on her bed waiting for the scores to appear on her smartphone. For many top students like Destiny, the scores might seem an academic footnote; she already had her diploma and had been admitted to college. Yet the idea that she might not have succeeded on the AP tests made her so anxious that, just as the scores began to download, she turned her phone face down, unable to look.

Destiny was experiencing the pressures of being a pioneer on the frontier of Advanced Placement, one of thousands of minority, low-income students being targeted for a nationwide expansion of the rigorous college-level courses. She took a deep breath, turned her phone back over and looked at the three numbers on the screen.

For such students, the scores show how well their education prepared them for college and whether they might earn college course credits, potentially saving thousands of dollars in tuition. For federal and state education officials who have invested $400 million in taxpayer dollars over the past decade to subsidize AP exams for bright, low-income students, the stakes are even higher.

So far, the expansion has not lived up to its promise. It has not delivered vast numbers of students from low-performing high schools to selective colleges with credits in their pockets, helping to bridge the academic gulf between the nation's rich and poor. Too often, students who haven't been prepared in earlier grades flounder in AP classes, or are awarded A's and B's in the courses and then fail the AP exams.

The high grades for course work can lull students into a false sense of security, said Steve Syverson, a board member of the National Association of College Admission Counseling and a former dean of admissions at Lawrence University in Wisconsin. Many students arrive at college with AP courses on their transcripts, but with skills so low they must take remedial classes.

"The kids are ... doing what society is telling them to do," he said. "We just set those kids up for complete failure because they just get hammered when they get to college."

A Baltimore Sun analysis of test scores showed a troubling discrepancy between grades for AP course work and scores on the exams. In at least 19 high schools throughout the Baltimore region, more than half of the students who earned an A or B in an AP class failed the exam.

Failure rates of 75 percent on the exam were common at Woodlawn and other Maryland schools with large numbers of minority and low-income students. For the 2011-2012 school year, the most recent available data, about 40 percent of students who took an AP test in the nation failed. But nearly 75 percent of African-American students nationwide failed, and the pass rates for Latinos and low-income students are far below those for whites and Asians.

By expanding the reach of Advanced Placement, the College Board, which administers the program, has created a de facto national curriculum for high achievers, and Maryland is the leading participant in the U.S. More than half of the state's public school graduates now take an AP class and nearly 30 percent have passed at least one exam, the highest rate in the country. In that sense, Maryland schools have become a laboratory for the nation.

Despite that glowing record, a close look at the data shows that students at many schools struggle.

Trevor Packer, head of AP for the College Board, acknowledges that the program is being misused in some schools, with students taking classes before they are ready. For instance, he said, 20,000 African-American students in Maryland took AP exams last year, but the College Board predicted that only 2,000 had a strong chance of passing because of scores on other tests.

Still, Packer is reluctant to intercede in local decisions about which students should take the classes. The College Board believes that most students still benefit from AP and should have that opportunity, he said, adding that there is no precise way to predict which individuals will succeed.

The exams are lucrative for the College Board, the New York-based nonprofit that already has a huge impact on the post-graduation ambitions of American high schoolers with its SAT test and CSS-Profile for financial aid. The organization reported total revenue of $720 million in 2011, the latest available, with AP and the PSAT test generating more than half of that figure. Packer declined to break out revenues from AP, but said that money is invested in the program to improve results.

Helping to drive the use of AP, federal officials sent Maryland $589,000 to subsidize exam fees for low-income students in 2012. County governments spent another $451,700 on the fees, and principals pulled money out of their budgets to help students who couldn't pay the fees but weren't poor enough to qualify for a subsidy. Most of the $89 exam fees, though, are paid out of family budgets.

A rough start

Destiny felt a bit intimidated last August as she first opened the squeaky, heavy metal door of Brian Patterson's meticulously organized biology classroom at Woodlawn. Only 14 students had signed up for the double-period class, which required mastery of complex material and lots of homework. She had looked forward to the class because she wanted to major in biology in college, but she knew Patterson was a tough grader.

The eldest of five children in a family centered by faith and church, Destiny hoped to be the first in her immediate family to graduate from college. With above-average SAT scores (between 530 and 600 on each section) and good grades, Destiny was the type of student selective colleges were seeking. She dreamed of a career in medicine or the sciences.

If she could get high scores on her AP exams in May, the college she chose might accept the credits and allow her to graduate earlier with less debt. Her mother, who was trying to take a few community college classes while juggling family duties, and her stepfather, a truck driver, did not want her to take on any debt.

But finding time to study was tough. On top of a heavy academic schedule that included three AP courses, Destiny's week was jammed with sports, duties as senior class secretary, church and a part-time job at McDonald's.