LOS ANGELES — Hundreds of photos of standardized tests have begun to appear on social-networking sites in California, raising concerns about test security and cheating by students.

In the worst-case scenario, the photos could lead to invalidating test scores for entire schools or prevent the state from using certain tests. For now, officials have warned school districts to heighten test security and investigate breaches. Students are not allowed to have access to cell phones or other devices that can take pictures when the tests are administered.

“Test security was compromised when students posted images of actual test questions, answer documents and test booklets to social networking sites,” Deb Sigman, a state Department of Education deputy superintendent, said in a letter Friday to school districts. “You have a responsibility to prevent any such incidences in the future.”

Educators involved in testing sign an affidavit asserting that they follow and enforce all rules.

Statewide, districts are about halfway through annual California Standards Tests, which make up most of a school’s state rating on the Academic Performance Index. The tests also are used in federal accountability systems.

The federal evaluation determines whether a school is subject to such sanctions as being closed or restaffed. A school’s state API score gets the most attention locally, with the highest ratings able to boost real estate values because parents will seek out those campuses.

But a school can lose its score entirely if 5 percent of tests are invalidated. Cheating or lesser mistakes by teachers and other staff led to canceled scores at about two dozen California schools last year.

Students typically don’t have anything at stake on the annual tests: Colleges don’t look at student scores, and the scores don’t affect course grades or grade-point averages — although L.A. Unified has experimented with rewarding improvement on a standardized test with a higher course grade.

So far, officials have found more than 100 students to be involved, but say they weren’t necessarily trying to cheat.

“Most of the images discovered so far contained only students posing with a closed test booklet, blank answer documents or answer documents with a message written on them,” the state Education Department said in a release. Some images, however, “appeared to contain test questions or completed answer documents” from both the annual achievement tests and the state’s high school exit exam, which is required for graduation.

Any picture could be evidence of a security breach, because a student with a cell phone could, for example, also search the Web for answers to a question, said Paul Hefner, communications director for the California Department of Education.

Woodland Unified, near Sacramento, first spotted the images. So far, 34 districts could have problems, including some in Southern California, although Hefner was not able to provide a list Friday.

Los Angeles schools Superintendent John Deasy said he doesn’t know yet if students from his district were involved.

“This could be a serious breach of security, and could compromise this year’s assessments,” he said.

State officials haves asked website operators to remove the images, which were posted on sites such as Facebook, Instagram, Pinterest, Twitter, Tumblr and Webstagram.