Progress reports without a grade. That’s what Mayor-elect Bill de Blasio proposed during the campaign and it led SchoolBook to wonder: what are all those numbers behind the grades and what can we learn from them going forward?

Under the Bloomberg administration, a school earned both a letter grade and an accompanying score, based on a methodology of student performance, the school’s environment, student performance and progress. But, for the most part, people just paid attention to the grade.

Shael Polakow-Suransky, the chief academic officer at the Department of Education, routinely encouraged journalists to report the ways the city included additional factors in its rating system, things such as how well eighth graders performed in high school. Now, facing the prospect of no more grades, we have done just that.

Below, SchoolBook explains how the city measured criteria to come up with the grades — and the scores.

These two charts show the distribution of letter grades for high schools and the distribution of overall scores. You can see that about two-thirds of high schools earned an A or a B in the 2012-2013 progress reports. In this year’s reports, overall scores ranged from as low as 24.5 to 105.9 for high schools. An overall score of 70 or higher got an “A;” 58 to 69.9 got a “B.”

Half of the high schools got an overall score between 54 and 73. The same trend is seen in other progress reports since the 2010-2011 reports. That’s when the city roughly set the percentage of schools that would receive each letter grade.

About 25 percent of all elementary and middle schools received an “A” in the past three reports. But what does that mean to parents or students?

It means that by the D.O.E.’s own formula, most schools are in the middle, not on one end of the spectrum. A school like Leadership and Public Service High School was given a B in this year’s progress report, while Bronx School of Law and Finance was given a C. Yet, the schools were within one tenth of a point for their overall score, 58 vs. 57.9.

The Bronx school even outperformed the Manhattan school in a couple areas: student performance and school environment, for example. The core difference between the two, however, was found in “College and Career Readiness.”

  • Leadership’s score in that area: 6.8 out of 10.
  • Bronx School of Law and Finance: 5 out of 10.

That metric is supposed to measure how well schools prepare students for “life after high school” and the department changed how it’s calculated this year.

Another example to illustrate the data in progress reports can be found at P.S. 15 Roberto Clemente in Manhattan’s Alphabet City. P.S. 15 had its second best progress report overall compared to other elementary schools, but its letter grade of C hasn’t changed in the last four years.

How could that be? The most recent progress report in particular shows how a letter grade for schools like P.S. 15 could be deceptive. The DOE gave P.S. 15 an overall score of 48.7 — one decimal from the cutoff for an overall grade of “B.” The school was determined to be in the 39th percentile of all elementary schools.

The school showed its biggest growth in “student progress” between 2012-2013 progress report and 2011-2012 progress report. That measures whether students improved between years on the English and Math tests compared to other schools. The2013 test scores in Math and English for 3 to 8 grades went down across the city and state, because it was the first exams aligned to Common Core, the state’s new learning standards. Even though the students at P.S. 15 improved on the tests compared to other schools, the schools progress report score for “student performance,” which measures how students performed on the tests, was down in the most recent progress report compared to the previous year.

The last time P.S. 15 received a “B” was in 2009. That year, however, P.S. 15’s overall score was deemed to be in the third percentile which means the city determined it had only performed better than 3 percent of all elementary schools that year.

P.S. 15 never received an overall score lower than in the 12th percentile of all elementary schools since then.

Perhaps anticipating the school grades debate will continue after the current leadership leaves office, D.O.E. officials issued a white paper on school accountability going forward, which highlighted recent changes made to the progress reports.