IT IS DISTRESSING to watch the debate over Boston exam school admissions divide the city.  The focus on how to allocate a fixed number of seats has created a zero-sum battle that is pitting neighborhood against neighborhood and racial and ethnic groups against one another.

The policy discussion needs to be reframed around providing more Boston students with rigorous college-preparatory high school experiences now, as well as setting up an incentive structure so that the number of students receiving such an experience increases over time.

The most impactful research the Rappaport Institute for Greater Boston has published in my six years as director is Joshua Goodman and Melanie Rucinski’s study showing how the racial and ethnic composition of the exam schools would change if students were evaluated using a test based on the actual curriculum they have been taught rather than the independent school exam.

The study found that such a step would increase Black and Hispanic enrollment at the Boston Latin School by nearly 50 percent and concluded that “there appears to be little tension between substantially improving diversity at BLS and maintaining the school’s rigorous academic standards.”  But the scope of that study was limited to how to make the Boston Latin School student body more diverse and did not address how to increase the total number of Boston students who receive a rigorous high school education.

Here’s an alternative approach that breaks free from the constraints created by having children compete for a fixed number of exam school seats.

First, add a fourth exam school as soon as possible, so that more students can learn alongside other similarly prepared students.

Second, have a rigorous readiness standard for who is eligible to attend an exam school.  Something like an A- grade average plus a top score on the MCAS or some other test that is based on the actual curriculum taught in the Boston Public Schools. The test score requirement would be a fixed and preannounced level, not a percentile among test takers.

Third, guarantee every student who meets the readiness standard a spot in an exam school. No student who is ready for a rigorous curriculum should be denied access.

Fourth, allocate eligible students among the exam schools through a choice-based lottery that allows family preferences among the schools to be reflected. All students meeting the readiness standard would have an equal chance of matching to their first-choice exam school.

Fifth, locate the new exam school in a physical building that will allow it to expand the number of classrooms as more students meet the readiness standard over time.

Sixth, guarantee that a fifth exam school will be created whenever the number of students meeting the readiness standard reaches a certain number.

By expanding slots and guaranteeing admission to an exam school for qualified students, this approach eliminates the current divisive dynamic where one student doing better on a test means another student loses access to the highest quality education. By creating a pre-announced threshold for exam school admission, it gives every family and school a clear and high standard to aim for. Most importantly, this approach creates a path that will allow the city to offer more and more students a rigorous high school education every year. If combined with substantial additional investments in career and technical education, a decade from now there could be 10 excellent high schools that Boston parents are striving to get their children into rather than three.

The hardest question is how to determine readiness. Any reliance on test scores has the potential to disadvantage students who have had fewer resources at home and at school, who have not grown up in the dominant culture, who have received less test prep, or who simply have one bad day.

But relying too much on grades is also problematic, given the difficulty of maintaining consistent grading practices across schools and over time. Indeed, a weighting system that appears on paper to be 70 percent grades and 30 percent test scores could in practice turn out to be nearly 100 percent on test scores if grade inflation results in most applicants having a 4.0 GPA.

And then there is the question of whether to give extra points to students who have grown up in more difficult circumstances and who have arguably achieved more even if their test scores are not quite as high.

Here’s my view. Almost any weighting among test scores, grades, and socioeconomic status can work so long as the overall framework maintains a high standard for readiness, offers an exam school slot to every qualified student, and creates a path for more and more students to receive a rigorous college-preparatory high school experience each year. Conversely, if Boston maintains a system that pits neighborhoods and families against one another in a battle for a fixed number of seats, there is no weighting scheme that will leave the city unscathed.

Jeffrey Liebman is the Malcolm Wiener \professor of public policy at the Harvard Kennedy School and director of the Rappaport Institute for Greater Boston.