Houston became the largest school district in the country on Thursday to adopt a merit pay plan for teachers that focuses on students' tests scores.

By a 9-0 vote, the Houston school board approved a plan that offers teachers up to $3,000 in extra pay if their students show improvement on state and national tests. The program could be expanded to provide up to $10,000 in merit pay for teachers.

The vote came after several teachers told the board during its monthly meeting they believed the plan was flawed and unfair because some teachers will be eligible for larger bonuses than others.

"This is not a perfect plan but it is a beginning," said school board president Diana Davila.

Other school districts around the country have implemented various types of incentive pay programs for teachers in recent years. Denver adopted one in November, becoming at the time the largest school district to do so. Houston, with more than 200,000 students, is the nation's seventh-largest school district.

The plan is divided into three sections, with as much as $1,000 in bonus pay in each.

The first will award bonuses to all teachers in schools rated acceptable or higher, based on scores on the state's main standardized test. The second ties pay to student improvement on a standardized test that compares performance to nationwide norms.

In the third section, reading and math teachers whose students fare well compared with others in the district would be eligible for bonuses.

The teachers' union doesn't approve of the plan, saying it focuses too much on test scores and is too complicated.

In general, teachers across the country have been paid based on their years of experience and education levels. Starting teachers in Houston make about $36,000 a year. The average salary in the district is about $45,000.