For a long time, Fairfax County has had the largest, and I think best, school district in the Washington area, and one of the best in the country. But now it has taken a bad turn. It is diluting challenges for high school students and junking a system that helps identify its best teachers.

Twenty years ago, Fairfax was the first in the area to require that Advanced Placement and International Baccalaureate students take the exams at the end of those courses. Fairfax School Board members understood then that those who did not take the final exams would not get the full college-level experience that AP and IB were designed for.

They also knew the tests emphasized essay questions written and graded by outside experts, and thus could not be dumbed down by teachers. The results revealed which teachers were best at preparing disadvantaged students challenged by such material.

The current School Board has decided to save money by letting students opt out of the tests. The district said it will still pay fees for the first six AP or IB tests each student takes. It also promised to encourage all AP and IB students to take the exams, but many Fairfax educators told me they doubt that will happen.

Board members appeared reluctant to discuss with me the effect of their decision on students who need a full jolt of AP or IB to be ready for college. School Board Chair Jane Strauss said the state would not allow the district to require tests unless it paid all test fees, and it could no longer afford that. "Our approach is to trust families to make decisions based on what they feel is the best for their child," she said.

That overlooks the fact that busy parents, particularly those not fluent in English, often let their children determine which high school challenges to accept. Sixteen-year-olds will be deciding whether to take tests that are three to five hours long.

Strauss declined to deal with the fact that even students who fail the AP and IB exams tend to complete college at higher rates, perhaps because they have been forced to struggle with those subjects in high school. A 2013 study by College Board researchers Krista D. Mattern, Jessica P. Marini and Emily J. Shaw, based on a sample of 678,305 students, found that "regardless of what score was earned on the AP Exam(s), students who took an AP Exam were more likely to graduate in four years or fewer than students who took no AP Exams."

Fairfax hopes to save $600,000, about two-hundredths of 1 percent of the budget, by making AP and IB tests optional. They would do better cutting back on the purchase of laptops, which do not appear to have the same good effect on learning as AP and IB tests do.

In Northern Virginia, the schools of Arlington and Prince William counties and the cities of Alexandria and Falls Church still pay all the fees and require that AP and IB students take the tests. "Allowing students to opt out of the exam could create two different levels of expectations in the same classroom and can have the effect of hollowing out the depth and rigor of the course experience," said Kenneth Bassett, director of student learning in Prince William.

Arlington's Washington-Lee High School, where 33 percent of students are from low-income families, has achieved the rare feat of every student taking at least one AP or IB course and test before graduation. Principal Gregg Robertson remembered one student saying he didn't think he was college material until he was required to take an AP course and test and discovered he could do it.

Fairfax County just installed a new schools superintendent, Scott Brabrand. He did well with AP as principal of Fairfax High School a decade ago. Thankfully, he told me if he sees a significant drop in test participation, he will ask the board to revisit the issue.