The Louisiana Department of Education has failed to implement state-required academic guidelines for deciding whether charter school contracts should be renewed, Legislative Auditor Daryl Purpera said Monday.

The agency relies on an annual school performance score, or SPS, rather than reviewing standardized test score results as required by state law, Purpera said in an audit.

Gains in school performance scores for middle and high school students do not ensure that standardized test score results are up because an SPS includes issues other than test results, like graduation rates and how many credits students have earned, the study says.

The report said 39 percent of the 18 charter schools examined whose contracts were renewed failed to show gains in standardized test results.

By doing so, the audit said, there is a risk the department "is renewing schools that may not have demonstrated improvement in the academic performance of its students, which is required for a school to be renewed."

A department official said those schools actually showed gains, which were included in the SPS calculation.

In their written response, department officials generally agreed with the two key findings in the report.

However, state Superintendent of Education John White disputed part of the study.

White said said "Louisiana has a clear and unambiguous history of using students' improvement on standardized test scores to determine whether to renew charter schools."

The report marks the second time in the past week that Purpera has faulted the the department's oversight of charter schools.

Charter schools are public schools run by non-governmental boards.

About 48,000 students attend the type schools reviewed.

When the schools are up for renewal, department officials check their operations and make recommendations to the state Board of Elementary and Secondary Education, which has the final say.

School performance scores are made up entirely of results on key tests for elementary charter schools.

For middle schools, exam results make up 95 percent of the SPS and 50 percent for high school students.

Purpera said in his audit that state law says "no charter shall be renewed unless the charter renewal applicant can demonstrate, using standardized test scores, improvements in the academic performance of pupils over the term of the charter school's existence."

BESE rules do not spell out how department officials should determine academic performance.

The audit reviewed schools considered between 2011-16.

During that period, 58 of 64 charter school contracts were renewed.

In the case of one high school, it said, the charter was renewed for three years despite worsening academic performance.

Other factors, including the school's graduation rate, bumped the score.

Follow Will Sentell on Twitter, @WillSentell.