Chicago Public Schools acknowledged Thursday that its graduation rate – heralded by Mayor Rahm Emanuel during his re-election campaign because the number of students finishing high school was reportedly rising – was overstated.

The school system was forced to make the embarrassing revision following a Better Government Association/WBEZ investigation earlier this year that found thousands of students were being counted as transfers when they should have been counted as dropouts. Transfers aren’t factored into the graduation rate, but dropouts are.

CPS previously claimed that 69.4 percent of students who started high school in 2009 graduated by the summer of 2014. But on Thursday, officials revealed that the rate is, in fact, 66.3 percent.

Asked whether the numbers were fudged on purpose, CPS Chief Education Officer Janice Jackson said when she was a principal, she knew how to properly categorize transfers.

Jackson also acknowledged that some principals might feel pressure to boost their graduation rate on paper so their school ranking is better.

“We have to make sure that accountability comes with support,” Jackson said. “What I can say is that I am encouraged by the upward trajectory, despite the changes we had to make.”

CPS announced Thursday that its 2015 graduation rate rose to 69.9 percent. However, only 84 more students graduated by summer 2015 than the previous year – the smallest increase in years.

CPS also revised the graduation rate for 2013, from 65.4 percent to 62.5 percent; for 2012, from 61.2 percent to 59.3 percent, and for 2011, from 58.3 percent to 56.9 percent.

CPS is now accurately counting transfers and dropouts, Jackson said.

Emanuel has said he aims to get the graduation rate up to 85 percent.

The school system hasn’t yet released the school-by-school graduation breakdown for 2015.

The BGA and WBEZ reported in June that a review of CPS’ own records showed at least 2,200 students from 25 Chicago high schools were wrongly counted as “transfers” – departing the system for another school district from 2011 to 2014 – when they should have been considered “dropouts.”

CPS officials initially said they had no plans to go back and adjust the numbers. It’s unclear why they changed minds.

This story was written and reported by the Better Government Association’s Sarah Karp, who can be reached at skarp@bettergov.org.