The Dallas school district said Tuesday that answers were changed on the math TAKS tests at Lang Middle School in April.
The Dallas Independent School District's investigation found that the testing irregularities didn't occur while students were taking the test.
The district said the eighth-grade math test answer sheets were not properly secured. There was "ample time" for the answer sheets to be changed after students took the exam, the district said in a statement.
According to the investigation, someone changed the answers while the tests were left in an unsecured room overnight, the Dallas Morning News reported.
But the district said it did not know who changed the answers.
The district said Lang's former principal and testing coordinator have resigned from the DISD. Neither have responded to requests from DISD investigators, the district said.
DISD's report was sent to the Texas Education Agency on Tuesday.
The TEA ordered about 407 eighth-graders at Lang to retake the math test in June, but most did not show up for it.
A committee evaluated about 250 students to decide if they should move on to the ninth grade. Twelve were held back because of their overall performance, the Dallas Morning News reported.