The District is invalidating standardized test scores of 18 testing groups at 11 schools after "serious test security violations" were discovered, signifying that cheating likely occurred at the schools, officials announced Friday.

The D.C. Office of the State Superintendent of Education hired the consulting firm Alvarez and Marsal to look at 2012 math and reading scores on the D.C. Comprehensive Assessment System (CAS) standardized tests across all 243 public schools.

The investigation flagged incidents of significant score increases or decreases from previous years, wrong answers that were changed to right answers and wide variation in student performance in a single classroom or school at 25 public schools, including 10 D.C. Public Schools and 15 public charter schools. Of the 25, 11 had serious violations in 18 testing groups -- which could be single classrooms or groups of classrooms, overseen by one test administrator. Four had moderate violations -- "anomalies with defined violations but not test tampering" -- and one school had a procedural problem. Nine turned up no violations.

The schools with some classes' test scores being invalidated are Beers Elementary, Brightwood Education Campus, Hendley Elementary, Kenilworth Elementary, Langdon Elementary, Miner Elementary, Winston Education Campus, Arts and Technology Academy Public Charter School, Community Academy-Amos I Public Charter School, Hope Community-Lamond Public Charter School and Meridian Public Charter School.

Friday's announcement comes on the heels of the revelation that D.C. officials were aware of possible widespread cheating in 2009, under then-Chancellor Michelle Rhee. An internal memo, obtained by PBS's John Merrow, shows an analyst's findings that 191 teachers at 70 schools may have replaced their students' wrong answers with right ones.

The District's investigation at the time resulted in one teacher being fired.