Cesar E. Chavez High School in Delano has been stripped of a closely watched index ranking of academic achievement after the school reported a violation of standardized testing rules.

The problem was with the Chemistry Standards Test, one of several tests that comprise the annual Standardized Testing and Reporting Program, better known as the STAR test.

A chemistry teacher at the school skimmed the test booklet in 2012 and committed some questions to memory, said Delano Joint Union High School District Assistant Superintendent James Hay. The teacher then used some of that information to help prepare students for the 2013 test.

The teacher didn't realize that doing that was a violation and had no intention of cheating, Hay said.

"It was one teacher, an isolated individual, and it took us all of about three seconds to decide to report that to ETS when we found out," said Hay, referring to Educational Testing Service, the nonprofit organization that administers standardized tests for the California Department of Education.

Chavez turned itself in, he said, because the school wanted to be certain "the integrity of our testing program was above reproach."

The identity of the teacher has not been disclosed, and the district said it could not reveal whether or not the teacher was disciplined due to personnel confidentiality policies. Hay did say, however, that the teacher is still employed and still in the classroom.

Results from the STAR test are used in a formula that determines a school's Academic Performance Index ranking. A school that scores above 800 on the index is said to be meeting state standards. Schools that score below 800 are given a timeline for raising their test scored and penalized if they fail to do so.

Chavez's most recent API score was 840.

The school omitted scores from the chemistry portion of the STAR test when it submitted its results to the state. As a result of that irregularity, Chavez will not have an API ranking for 2013 and will not be eligible for certain awards for the 2013-14 school year.

Only about a fifth of STAR test questions are changed every year, said Tina Jung, a spokeswoman for the California Department of Education.

Teachers are not supposed to read the test booklet or reveal the previous year's questions; neither are they supposed to prep students for the STAR exam, which is designed to measure cumulative knowledge for the entire school year, Jung said.

Hay said administrators all agreed it would be better to take a hit this year than do anything that might cast doubt on the school's previous or future academic accolades.