Tomorrow, when the city releases its progress reports for elementary and middle schools, parents will begin the annual rite of deciphering their schools’ report cards. But this year the tradition will be complicated by a new formula and, for many schools, lower grades.

The city is trying to accomplish several goals at once: It is hoping to improve the methods it uses to measure student progress and reduce the wild fluctuation and inflation of grades that has marked past years’ progress reports. At the same time, city officials hope to convince parents, teachers and principals that the grades are meaningful, especially in light of this year’s sharp drop in test scores across the city.

Last year, the city gave 84 percent of elementary and middle schools A’s, while 13 percent received a B, and 2 percent received a C. Just five schools were given D’s, and two were given F’s. Those grades were much higher than the year before, when 38 percent of schools were given an A. In 2007, when the reports were first issued, 23 percent received that rating.

For this year’s progress reports, the city is making several big changes to how the grades are calculated. First, it is modifying how the city calculates students’ progress. In the past, a significant percentage of a school’s grade  — 85 percent for elementary and middle schools — was based on student performance on state math and reading scores. So when test scores went up throughout the city in 2009 (reflecting a statewide trend), the grades soared on progress reports.

This year the city is doing something different. It is comparing the progress of each student to other students who began the school year performing at the same level.

In other words, students who scored a level 3 on the math exam in 2009 will have their 2010 math scores judged against other students who scored similarly the year before. If one student ends the year with a higher score than another student who started at the same place, the first student’s school will receive more credit.

“It’s still kid to kid, but kid to kids who started at the same place,” Shael Polakow-Suransky, the city’s deputy chancellor for accountability, told WNYC’s Beth Fertig. “It’s a much more precise measure.”

The new formula is based on Colorado’s “growth model” for measuring student progress.

In addition, the city is distributing progress report grades across a curve for the first time. Only a quarter of schools will be granted A’s, and roughly two-thirds will receive either a B or C.

Knowing that state education officials planned to make the tests more difficult to pass, the city announced the curve in January. But when the drop in test scores was more severe than the city had anticipated, officials decided to add a floor to how far schools’ progress report grades can fall.

Under this year’s curve, 15 percent of schools are slated to receive the lowest two grades. High stakes are attached to the progress report grades. Grades of D or F, as well as a string of three C’s in a row, trigger scrutiny from the city, which can frequently lead to a school being restructured or closed.

Critics of the school accountability system who charge that the reports rely too heavily on the state exams are not likely to be assuaged by the changes. Students’ scores on the standardized tests still account for 85 percent of a school’s total grade. The rest is determined by reviews of the school environment and the results of surveys given to students, parents and teachers.

Some principals also remain wary of how their students’ progress is compared to that of students at other schools. The city has historically assigned schools “peer groups” for the purpose of comparison, and some principals have complained that those groups of schools are not as similar as the city says.

The city is also making a number of smaller changes to this year’s progress report grading. For middle schools, a pilot program will factor students’ class grades into schools’ progress reports, as the city does for high school report cards.

And schools will be given more credit this year for closing the performance gap between their general education students, their special education students, and those learning English. Also for the first time this year, schools in the city’s District 75 program, which serves the most severely disabled students, will also receive progress reports.

High schools are expected to receive their progress reports in November. The city is not changing how high school grades will be calculated this year.