A parent asked me if trouble at Loudoun Valley High School in Loudoun County meant her daughter’s grades were being inflated. A detailed Oct. 30 article by Danielle Nadler in Leesburg Today said Loudoun Valley Principal Sue Ross was being investigated for allegedly pressuring teachers not to give out bad grades. “The C is the new F at our school,” one teacher was quoted as saying.

Grade inflation in U.S. schools is both troublesome and common. The parent was right to worry. We don’t want our children entering college or the workplace wrongly thinking they can handle the challenge because of lies on their report cards. Fortunately, there are ways for parents to check their school’s commitment to reality.

In this region, college-level courses like Advanced Placement and International Baccalaureate have become so pervasive at so many schools that they provide an independent benchmark on grade inflation. In Loudoun County, like several other Washington-area districts, a majority of students take AP, often in several different subjects.

AP teachers can still give a class grade of A to a weak student, but they are less likely to do so because there’s an independent AP exam. If that student does poorly on the AP exam in a typical suburban school, the teacher will be embarrassed and perhaps lose the AP class. (This does not apply to low-income urban schools that think the challenge is good for students even though very few will score well on AP exams.)

Parents can compare the report card grade to the AP test score and know inflation has likely occurred if the former is significantly higher than the latter. One study says a 4 or 5 on an AP exam is equivalent to a high school A, a score of 3 is similar to a B and a 2 is comparable to a C.

Because so many of its students take AP, Loudoun Valley High is one of the least susceptible schools to grade tampering. Its participation rate on AP exams last May was in about the top 2 percent of schools nationally, based on the data I collect for The Washington Post’s annual America’s Most Challenging High Schools list. In 2012, more than 60 percent of Loudoun Valley’s graduating seniors got a 3 or better on at least one AP exam, three times the national average.

Loudoun County high school Web sites have much AP data. The AP participation rates for most local schools, and more than 2,000 schools nationally, can be found at washingtonpost.com/local/highschoolchallenge.

Since Ross was made principal in 2005, the school’s AP participation rate has increased 77 percent. The school’s average SAT score also jumped from 1634 to 1659 in the past three years, a good sign because colleges often look at SAT scores to confirm grade accuracy.

AP exams are written and graded by outside experts, such as experienced teachers and college faculty. They don’t know the students they are grading and have no incentive to give high marks to those who don’t deserve them. Most other high school exams are graded by the teachers of those classes, who are sometimes pressured by students, parents and principals to raise the marks.

Principals may have good cause to discourage low grades from teachers who they think are not working hard enough to help students improve. The legendary AP teacher Jaime Escalante was enraged by colleagues who gave struggling students F’s the first semester, leading many to drop the course. He thought they deserved more encouragement and time to learn. But principals also have told teachers to change grades simply to mollify angry parents.

I am not sure what is happening at Loudoun Valley High. Parents should compare grades to AP scores and see for themselves.

For previous columns, go to washingtonpost.com/class-struggle.