"We are sorry to report this, but at the same time, too, we want everyone to know what actually has happened for this testing round," Britz said.
School officials would not confirm the teacher's name, citing personnel issues.
McKinley students began taking the standardized tests, known as STAR exams, last week.
A third-grade student reportedly told McKinley's principal, Bobbie Kavanaugh, on April 16 that a teacher had helped third-grade students with the test questions, according to the district.
Sharon Cuseo, director of instruction, investigated the case alongside Tom Kissinger, director of elementary education for Burbank Unified. They reported their findings to the California Department of Education this week.
"We think it's a likelihood that a class set of tests will be invalid. What happens from there, we really do not know," Britz said.
When reached Friday, a representative for the state Department of Education declined to comment.
The STAR results — as well as those of other standardized tests — make up the API scores for individual schools and districts as a whole.
In 2012, McKinley's API was 835, comfortably above the state's 800 target score.
Except for school board President Larry Applebaum, no other board member spoke publicly about the alleged cheating.
"From time to time, things happen, and we do the best we can," Applebaum said. "I'm appreciative of staffs' diligence in pursuing what was an investigation and doing the right thing."
Follow Kelly Corrigan on Twitter: @kellymcorrigan.