A tougher grading scale on the District’s 2013 standardized tests would have yielded lower-than-reported math proficiency rates for many schools, with stark differences at the middle-school level, according to data released Monday by the Office of the State Superintendent of Education.

But reading proficiency rates at most D.C. schools would have been higher than reported had city officials used the alternative grading scale, which educators recommended to judge students’ proficiency on more rigorous tests aligned with the Common Core State Standards. City officials instead discarded the alternative scoring and went with a model they said allowed them to compare scores across years, a decision that resulted in gains in math and reading.

Citywide, the alternative grading scale would have yielded math proficiency rates that were 3.6 points lower than in 2012, and it would have shown reading proficiency rates that were 6.6 points higher. But the OSSE emphasized that the results are unofficial and — because they emerge from a different grading scale — cannot be compared with proficiency rates reported in prior years.

“These unofficial proficiency percentages should in no way be compared to results from previous years, but should be considered the creation of a new baseline or measure that stands alone,” wrote Emily Durso, the interim state superintendent of education, in an e-mail to school leaders Friday.

The OSSE released the recalculated test results in the wake of a Washington Post report that revealed city officials’ decision to reject the alternative grading scale after seeing how it would affect results. Agency officials said their choice was meant to avoid wreaking havoc on charter-school rankings and other accountability systems.

The OSSE’s failure to publicly explain that decision led to fierce criticism and accusations of cheating from David A. Catania (I-At Large), chairman of the D.C. Council’s Education Committee.

“I’m delighted that these scores have been released,” he said Monday. “Now our students and teachers and parents and other professionals can look at the numbers honestly and evaluate how the students did on this tougher test.”

OSSE officials maintained that the test gains announced at a celebratory July news conference are the best measure of progress.

The alternative results offer a glimpse of the challenge awaiting schools in 2015, when the city is slated to administer a new and more difficult test aligned to the Common Core, new national standards for K-12 education. Scores are expected to drop.

D.C. middle schools appear to face the largest leap in math, according to the results. At Kelly Miller Middle School, for example — where Mayor Vincent C. Gray (D) announced the city’s test score gains this summer — 53 percent of students were deemed proficient in math, according to the grading scale the OSSE used. But only 37 percent of students were proficient under the rejected scale. Many middle schools saw similar discrepancies.

“As a district, we have many ways to measure how our students are performing,” Schools Chancellor Kaya Henderson wrote in an open letter to the community Monday. “All of these measures tell us the same thing. We have made a ton of progress and we have lots of work left to do.”

Under the rejected grading scale, 42.9 percent of the public school system’s students would have been deemed proficient in math, lower than the 49.5 percent reported in July. Meanwhile, 49.5 percent would have been proficient in reading, higher than the 47.4 percent previously reported.

Naomi DeVeaux, deputy director of the D.C. Public Charter School Board, pointed out that some schools — including Eagle Academy, E.L. Haynes and D.C. Preparatory Academy — scored higher under the alternative grading scale. Those results show that “some schools are already using instructional strategies and math curricula that maximizes the higher-order thinking skills required for students to truly master” the new Common Core standards, DeVeaux said in a statement.