The state has stripped National City Middle School of its Academic Performance Index scores after test regulators flagged it for an irregularity in testing procedure.
The scores measure performance and give parents a tool to evaluate how their child's school compares to others across the county and state.
National City Middle School is the only in the county to lose its performance measurement after the state flagged it for an "adult irregularity in testing procedure."
A Sweetwater spokesman said the incident occurred last Spring when a special-education teacher read aloud a passage meant to measure reading comprehension.
"It's not an instance of the teacher trying to help anybody. It wasn't the teacher trying to get them to score higher. It was a mistake. It was just a simple mistake," Spokesman Manuel Rubio said.
Rubio said a teacher is allowed to read some portions of the California Modified Assessment exam to some students with certain disabilities, but the teacher wasn't allowed to read it for all 35 students in the classroom.
He added that it was an isolated incident affecting only one classroom during this one school year.
Carmen Jimenz, a National City middle school parent who was picking up her child on Monday, said she was shocked to learn the news.
"That's messed up because they did so well last year. They were among the top," Jimenz said.
Between the 2010 and 2011 school years, the middle school recorded a hefty 34-point jump in test scores.
According to a state database, 23 other schools faced similar penalties statewide.