Florida's elementary and middle schools saw unprecedented — yet unsurprising — declines in their state-issued grades released Friday.

About 53 percent of schools earned lower grades than a year ago, state education commissioner Tony Bennett announced. Data show that 548 schools were protected from even bigger drops by a "safety net" rule preventing schools from sliding more than one letter grade.

Amid growing controversy over Florida's ever-changing quality measures, the state Board of Education imposed the safety net to help schools cope with shifting rules, tougher new tests and a transition to the rigorous Common Core State Standards.

"We are very confident in saying that decline in school grades is a result of increased expectations," Bennett said during a conference call with superintendents. He said he didn't expect grades to improve anytime soon, given that new standards and tests are on the way.

He broke the 2013 school grades down as follows:

• 760 A's, compared to 1,242 in 2012

• 677 B's, compared to 609 in 2012

• 721 C's, compared to 494 in 2012

• 353 D's, compared to 212 in 2012

• 107 F's, compared to 40 in 2012

Pinellas, Hillsborough, Pasco and Hernando all earned overall C grades. The new marks hit hardest in Pinellas, where eight schools received F grades, up from three last year. Just five Pinellas schools scored better than last year, and 50 posted lower grades. About half of Hillsborough schools improved their grades, while most of Pasco schools either remained flat or fell back, with two raising their grades.

Hernando had its first F school ever, at Eastside Elementary.

What do these grades mean to parents wondering about the quality of their children's schools? Because of the shifting standards, the grades aren't comparable to past scores.

But Bennett said the public could put stock in A grades, as they show high performance in the face of tougher criteria. On the other end, he said, the F's are equally valid because those schools couldn't even be helped by the safety net.

More than 150 schools escaped getting F's because of the net, including four in Pasco County, seven in Pinellas and 13 in Hillsborough.

Pinellas superintendent Mike Grego sought to give credit to schools that improved in the face of these challenges, citing Woodlawn Elementary's jump from D to B as an example.

"We as a state have done this cycle: doing better, better, better and then dropping back," Grego said. "This is one of those instances where we are dropping back, but this is not the time to not celebrate."

Given the low marks, though, Hillsborough School Board member Cindy Stuart worried about public perception.

"When people are looking at houses in an area, they do look at grades of the schools, not knowing that the whole thing is a shell game," Stuart said. "To a teacher who teaches in an A school, it is a big deal because they get that A-plus money."

Pasco Superintendent Kurt Browning accepted the grades as measures of how well each school met state grading criteria — and little more.

"We need to keep in mind that our schools are transitioning to new standards and we are stuck with an old assessment system that is not aligned to these new standards," he said in a release. "Additionally, the current grading system has once again undergone significant changes."

Hernando superintendent Lori Romano said the data behind the grades mattered more.

"Yes, we take stock in it, yes we use it to drive our work, but we have a lot of other factors that we use to determine the effectiveness of a school," said Romano.

Even before the grades came out, leaders across the state were making their criticisms known.

Miami-Dade superintendent Alberto Carvalho, who urged the Board of Education to protect districts from shifting standards, jabbed the system on Twitter.

"Through relentless and unscientific accountability manipulation, yesterday's A is today's C," he tweeted.

The Florida Democratic Party emailed supporters late Thursday to blame Gov. Rick Scott.

"Rick Scott has failed — well, we knew that already. His failed education plan is downright disgusting. … Since he came into office, the number of schools F rating has more than doubled," chairwoman Allison Tant wrote.

The state board agreed to the safety net in mid-July, after hearing complaints from Carvalho, Hillsborough superintendent MaryEllen Elia and others.

Initially skeptical, Bennett eventually backed the safety net.

Board member Sally Bradshaw, Jeb Bush's chief of staff when the grading system was devised, said during her tenure on the board, "all we have done is change rules to make calculations more acceptable to the system."

The board's 4-3 vote for the safety net was denounced by both the pro-accountability side, which said the state was masking the truth, and the anti-school grading side, which argued that there never had been any truth in an overly-manipulated system.

Bennett said he saw the safety net as the fairest way to keep school accountability moving forward while considering district concerns. He pledged not to offer any other changes to the grading system until a new model based on the Common Core and related tests is fully in place for 2015-16.

The elementary and middle school grades are based on the FCAT tests. High school grades, which include more factors, are not expected until late fall.

Staff writers Lisa Gartner, Marlene Sokol and Danny Valentine contributed to this story. Jeffrey S. Solochek can be reached at jsolochek@tampabay.com or on Twitter @jeffsolochek.