The tests cost $86 each, and up until last spring the St. Mary’s County public schools were paying for them. Unfortunately, though, hundreds of students weren’t doing well enough on Advanced Placement tests to earn college credit.

The idea behind having the schools pay for the tests initially was to encourage more students to take the academically rigorous AP classes, which are intended to parallel college-level work.

It worked. Enrollment in AP classes in each of the last two years was about 2,400. In 2009 AP enrollment was just shy of 1,700.

There are rewards for students taking these tougher classes. They learn to do the type of work they will be expected to do if they go on to college. And if they score well enough — a 3 or higher on a test that has a maximum score of 5 — many colleges will award them college credit. That means they will save hundreds of dollars in tuition costs that they would pay to take a similar class in college.

So the $86 investment for each test can have a big payoff.

But the problem was that hundreds of students in St. Mary’s were scoring below that level, so no college credit. Last spring the St. Mary’s schools stopped reimbursing students for the cost of the tests if they scored a 1 or a 2. Some were doodling on the paper or filling in random answers.

The new policy saved the public schools about $100,000 last spring. It also cut into the number of students willing to take the AP test in the first place by about 600. If there wasn’t a reasonable chance of earning a score high enough to give college credit, students and their parents reasoned, why take the test? But the actual number of tests that scored high enough to earn the credit rose by 44 to 1,151.

So it appears that the new policy has worked as intended. Enrollment in AP classes remains at a high-water mark. The number of students earning college credit has risen and the school system is saving money by not paying for tests when students do poorly.

There may be some students who don’t take the test just because they risk losing money; but there are safeguards in the policy for students likely to do well from families that truly can’t afford it.

Meanwhile those students who don’t take a test they were never going to score well on anyway haven’t lost anything. By taking the class in the first place they have still gained something of value. They have been in a classroom where the work is tough and demanding, and they have learned what it takes to do well.

The St. Mary’s policy is a compromise between paying for no tests and paying for them all. It still offers what amounts to a financial reward for hard work and achievement. But it doesn’t subsidize those who haven’t measured up.