UNION CITY -- James Logan High School has been stripped of its Academic Performance Index rating because it violated a state standardized testing rule this year, when administrators allowed students to complete parts of a test that they had started on a previous day.

New Haven Unified School District officials notified the state about the violation on May 9, a day after they learned about it while talking with Logan High administrators.

"Once we realized we had violated a guideline, we self-reported it," said Arlando Smith, New Haven Unified's chief academic officer and interim co-superintendent.

Wiping out the API score for Union City's largest campus could negatively affect its future. Schools that continually fail to meet API performance targets can be placed in Program Improvement, a category where state education officials work with schools to improve their test scores. If a campus does not adequately improve after five years, the state could take it over, said Tina Jung, a California Department of Education spokeswoman. "But we have never done that," Jung said.

Some of the 27 California schools recently stripped of API rating were cited for a "security breach." That included a Southern California instructor thought to have broken a rule by using past years' testing material as a study guide for students taking the California Standards Test, an exam that contributes to schools' API ratings.

The majority of campuses, including Logan High and five other Bay Area schools, were placed in the more benign category of "testing irregularities."

Logan High broke a rule when site administrators allowed 209 students "who had not completed various tests during the first sitting ... to complete tests on another day," according to a state education document called an "Irregularity Report Form."

Smith said the infraction was an honest error by instructors who did not know the test rules governing allotted time.

"There was no cheating involved; there was no attempt to deceive," he said.

California schools' scores recently were posted on the state Department of Education's website, where the API rating for Logan High and other cited campuses was left blank. A list of those offending schools and the details of their misconduct was first reported by the Los Angeles Times, which obtained school district reports through a public records request from state education officials.

The other Bay Area campuses cited by the state are: Ruskin Elementary in San Jose's Berryessa Union School District; Vallejo High in Vallejo City Unified; and Bay View, Westlake and De Laveaga elementary schools in the Santa Cruz Schools district.

New Haven Unified has about 13,000 students attending 11 K-12 campuses and an adult school in Union City and south Hayward.

In 2012, Logan High earned an API score of 737, which met state performance targets. This year, it would have shown growth had its score of 745 not been erased by the rule violation, New Haven Unified officials said.

To regain momentum on those scores next year, school district officials say they will make a concerted effort to prepare teachers and students better for testing rules.

"We will improve our communication, in terms of site preparation," Smith said. "We need to make sure we are clear on what's allowed and what's not allowed. Even if you've said something 10 times, it doesn't hurt to say it an 11th time."

Contact Chris De Benedetti at 510-353-7011. Follow him at Twitter.com/cdebenedetti.