Shielded from any nosedives, public school letter grades were largely unchanged for the 2013-14 school year, with 28 percent of schools receiving a D or F, the state Department of Education announced Tuesday.

However, state Superintendent of Education John White said schools will have to show improvements to meet higher expectations starting with the 2015-16 school year.

“We are transitioning to a much higher bar,” White told reporters during a 45-minute webinar.

Another 18 percent of schools received an A, up four percentage points over last year; 28 percent got a B and 26 percent received a C.

The Zachary school system is again rated tops in the state.

Last year, 28 percent of schools statewide were rated D or F.

Another 14 percent were rated A, 29 percent B and 29 percent C.

The static statewide results stem from a temporary policy approved in January by the state Board of Elementary and Secondary Education.

Under those temporary rules, the overall distribution of grades cannot differ from the 2012-13 results for the 2013-14 and 2014-15 school years.

BESE took the action amid fears that the move to Common Core and higher expectations would cause letter grades to plummet.

Some public school groups, including teacher unions and the Louisiana School Boards Association, had urged a freeze on letter grades during the transition.

The department said that, as a result of BESE’s policy, letter grades for 21 public schools in Louisiana rose.

The state has more than 1,300 public schools.

One notable change over last year was a 6 percent rise in the number of elementary schools that earned an A.

White said those schools made concerted efforts to raise scores from the third of five achievement levels — basic — to the fourth level — mastery, which will soon be the new state standard.

Schools will soon start earning fewer points for students who score basic, which was the standard set by state officials in 1999.

“That is what every school is going to have to do in the future or they are not going to be able to maintain higher letter grades,” he said of elementary schools that showed gains.

Scott Richard, executive director of the Louisiana School Boards Association, said while he did not want to diminish any improvements, the grading system is flawed. “It really begs the question as to the validity of the entire system when you start curving and tinkering with the system,” Richard said.

Schools are rated on a 150-point scale.

The state average is a B.

However, the standards are going up because Louisiana students typically score near the bottom nationally on more rigorous national standardized tests.

After Zachary, the top-scoring school districts included the Orleans Parish school system, which includes 20 schools, and the Central, Ascension and St. Charles school districts.

The letter grades, which stem from a 2010 state law, are based on the results of standardized exams.

The marks are designed to show parents and other taxpayers, in easy-to-understand terms, how public schools are faring.

The first results were issued in 2011 and showed that 44 percent of public schools statewide received a D or F.

Gov. Bobby Jindal, who pushed the law, used those findings to help push through sweeping changes in public school operations during the 2012 legislative session.

Critics contend the results are misleading.

The latest tally shows that the East Baton Rouge Parish school system got a C, the same as last year.

While the two-year BESE policy prevents the overall distribution of grades from declining those for individual schools can drop.

A total of 195 schools — about 15 percent — showed grade drops from the 2012-13 school year.

Follow Will Sentell on Twitter, @WillSentell. For more coverage of Louisiana government and politics, follow our Politics blog at http://blogs.theadvocate.com/politicsblog.