An A in science tells a parent: Your child knows her chemistry.

That A, however, could have been granted in part because the student often raised her hand in class and returned signed quizzes promptly. And those quizzes might have been far easier than those given in a school, or even a classroom, nearby.

The Montgomery County Board of Education, determined to make report cards mean the same thing in every county school, recently decided that grades should reflect how well a student has mastered official academic objectives, without points added for class participation, effort, improvement or other such factors.

Grades are to be "a judgment of what children know and are able to do based on the curriculum, and nothing else," said Barbara Haughey, principal of Ashburton Elementary School in Bethesda and co-chairman of the school system's grading committee.

Montgomery plans to pilot-test new report cards, as early as next school year, for grade levels where new curricula have been implemented. To begin to make grading consistent -- a quest few school systems have attempted, much less achieved -- the central office must create very specific guidelines for what constitutes mastery of each content standard, not to mention what exactly qualifies as A work, or B work, and so on.

Under the current policy, teachers are to grade based on "mastery of objectives," but it is not specified whether those objectives come from the curriculum or the teacher. "Some teachers give students credit for handing in their interim [report card] and getting it signed," said Sharon W. Cox (At Large), vice chairman of the school board. "How does that tell us they're learning what they need to know?"

The imprecision of grading in U.S. education has long troubled researchers, who cite standardized test results that have shown students in high-poverty schools with mostly A's and B's on their report cards scoring the same as C and D students in affluent schools. Only recently have educators begun to try to standardize grades, as a logical outgrowth of standardizing learning. But current grading reform usually involves steps like Prince William County is trying out: amplifying report cards with information about a child's competence on various standards.

Teachers across the country traditionally have had some leeway in deciding what kind of work rates an A or F. In general, they make up tests that assess the objectives as they see them, and they might tweak grades at the margins -- by giving points for participating in class discussion or handing in homework, by nudging a C to a B for the child who tries particularly hard.

Grades, in other words, have signified a bit more than a sum of a child's knowledge and academic abilities. Altering that will not be easy.

"It's really a big change in culture," said Robert J. Marzano, senior scholar at the nonprofit firm Midcontinent Research for Education and Learning in Aurora, Colo., and author of the book "Transforming Classroom Grading." "The hard part is not making it work conceptually. The hard part is people giving up their idiosyncratic grading practices."

The argument that lousy students can get by on charm alone is a red herring. It's already prohibited by Montgomery County schools, whose policy states: "Letter grades are not to be adjusted by personality factors, social achievement or deportment."

Nearly all teachers figure grades with a formula that, while of their choosing, is divided into test scores, classwork scores, homework scores and the like. There is no column on their spreadsheets for "sweet kid."

But teachers often factor in work habits. In a 1999 study of Virginia Beach schools, for example, 80 percent of teachers said that ideally, effort and behavior should be reported separately from academic achievement -- but two-fifths said that in reality, they include those factors in grades, mainly as an impetus for classroom control.

When implementation of Montgomery County's new policy begins, grades are to be an "accurate reflection of student achievement compared to grade level or course expectations outlined in the curriculum." Work habits will continue to be reported separately through middle school. As well, report cards might eventually include specifics about the objectives children are being graded on.

In the last few years, school systems that have converted to report cards explaining student progress category by category have found mixed results. Amy Jordan, principal of Ashland Elementary in Manassas, one of the Prince William schools pilot-testing new report cards, said that overall, teachers and parents like the new system. But New York City had barely begun using an elaborate, 10-page elementary school report card, which enumerated progress in more than 100 skills, when new Chancellor Joel I. Klein ditched it as too mystifying for parents and too taxing for teachers.

Montgomery's plan to link marks specifically to grade-level objectives has potentially complicated reverberations that seem to contradict the school system's widely promoted goal of individualized, effort-based instruction -- teaching each student on the level he or she is at and taking the time to help a struggling child both master the material and gain confidence that he can.

A fourth-grader who reads, say, at a second-grade level is taught lower-level fundamentals at the same time teachers try to catch him up on the fourth-grade skills. The quizzes and assignments by which a teacher judges that child now include work at both those levels.

Grading each child purely on grade-level standards means that the child who started far behind would be relegated to poor grades no matter how well he does the work he is taught, unless he aces tests of the fourth-grade standards.

To Bernard Rooney, a fourth-grade teacher at Rock Creek Forest Elementary School in Chevy Chase, the idea of having to grade a child on standards well above his level makes him uncomfortable. "A fourth-grade student working on the fifth-grade level would get all A's. A fourth-grade student on the third-grade level would get a C or D. Does the parent get a true picture of how the child is doing?"

"The message teachers are receiving is a very strong one: There are grade-level indicators for each grade," said Betsy Brown, co-chairman of the grading policy work group and Montgomery's program supervisor for English and language arts. Teachers should provide support for students who need it, she said, but when handing out grades, "they can't lower the floor for kids."

Montgomery County school board member Sharon W. Cox says teachers should use grades only to show what students know.