The District has voided the 2010 standardized test scores of three classrooms after an investigation found evidence or a strong suspicion of cheating, officials announced Wednesday.

The disclosure comes amid ongoing reviews of security questions related to the D.C. Comprehensive Assessment System, the high-stakes test used to measure student achievement, teacher effectiveness and annual progress as required by the federal No Child Left Behind act. It also comes as multiple school system and teachers’ union sources have said that at least two instructors were fired for inappropriate actions while administering tests.

Acting Chancellor Kaya Henderson declined to discuss the circumstances of the dismissals, citing privacy rules. Improper actions could involve giving students advance looks at test questions, prompting students during tests or tampering with answer sheets after the tests are completed.

D.C. test scores have been closely watched in recent years as school reform efforts have taken root. Rising scores on the city tests and on separate federal tests have been cited as signs of progress.

D.C. State Superintendent of Education Hosanna Mahaley said Wednesday that the troubles identified in 2010 test results were isolated. “Most of our teachers and students are playing by the rules,” she said.

Whether Wednesday’s action will dispel questions about the reliability of the city’s test scores remains unclear. A USA Today investigation published in March found that classrooms in more than 100 D.C. public schools showed higher-than-average rates of erasures from wrong to right answers on the annual tests between 2006 and 2010. They included schools that showed some of the most dramatic growth in scores. Henderson has asked the office of D.C. Inspector General Charles Willoughby to investigate the newspaper’s findings.

Testing experts regard elevated erasure rates as an indicator that students might have been improperly steered toward changing answers or that answer sheets were tampered with after the test was given. Classrooms in some schools, such as Noyes Education Campus, Bowen Elementary and Aiton Elementary, showed extraordinary levels of erasures: from 10 to 12 times the District average.

Mahaley, speaking Wednesday at the weekly news briefing of Mayor Vincent C. Gray (D), did not elaborate on what could have gone wrong in the three classrooms. Mahaley’s agency oversees the administration of the test — known as DC-CAS — in all public schools, traditional and charter.

In a letter to Henderson, Mahaley’s office noted that the three classrooms with voided scores were at Noyes, C.W. Harris Elementary and Leckie Elementary schools.

The invalidation of those scores came in response to answer sheet irregularities in classrooms at 18 schools — 10 in the school system and eight public charters — that were flagged after an analysis by CTB/McGraw Hill, the publisher of the test.

Mahaley’s office conveyed those preliminary findings to the D.C. school system, which hired Caveon, a Utah-based test security firm, to review security procedures and interview staff in the classrooms that had been flagged. Safiya Simmons, a spokeswoman for Henderson, said Caveon “concluded that two classrooms had possible testing irregularities and one classroom had a confirmed case of testing impropriety.”

Henderson’s office related the findings to Mahaley, who informed Henderson Wednesday morning of her decision to invalidate reading and math scores in the three classrooms.

Mahaley, who called Caveon’s assessment “three proven incidents of cheating,” said that in a school system with 3,800 classrooms and 45,000 students, the finding was testament to the effectiveness of the system’s security protocols.

Former schools chancellor Michelle A. Rhee, who resigned the post last fall, said in an e-mailed statement: “I believe this process validates the integrity of the vast majority of the tests taken in classrooms across DCPS.”

The D.C. public school investigation, led by Caveon, has coincided with at least two dismissals related to testing, according to school system and teachers’ union sources.

The sources said a fourth-grade teacher at Noyes was dismissed within the past two weeks for test security violations on the 2010 DC-CAS. It could not be learned exactly what the teacher was accused of or how the case fits into the broader questions being probed by Caveon, D.C. schools or the inspector general’s office.

Simmons confirmed that a fourth-grade teacher at the school recently left the city school system, but she would not comment further, citing confidentiality rules.

Another teacher, a special-ed instructor, was also dismissed within the past week, according to Washington Teachers’ Union President Nathan Saunders. Other school system sources said the second firing involved tests from another school year.

In an e-mail Wednesday, Henderson did not rule out further disciplinary actions.

“I have been consistently clear that DCPS is moving aggressively to take appropriate action, up to and including termination, in cases where any cheating has been confirmed and a teacher acted inappropriately,” Henderson wrote.

District officials also made public a list of test security incidents on the 2011 DC-CAS, which was administered last month. The state superintendent’s office said its security monitors found 31 irregularities in regular and charter schools, of which 14 will be subject to further action by District authorities. The list does not identify the schools.

A summary released by Mahaley’s office contained only brief descriptions of the incidents to be investigated. Among them: “inappropriate level of support given to students”; “three teachers in three classrooms provided aid to students”; and “proctor gestured and spoke to student during testing.”

Staff writer Michael Alison Chandler contributed to this report.