NASHVILLE (AP) - Tennessee's largest teachers' union is suing the Knox County Board of Education, claiming the student test scores used to assess a teacher's performance were flawed and cost her a bonus.

The Tennessee Education Association has long argued that the Tennessee Value-Added Assessment System, or TVAAS, shouldn't be relied upon because it's a statistical estimate.

The lawsuit says Knox County teacher Lisa Trout was unfairly denied the district's bonus after being misled about how her TVAAS estimate would be calculated.

TEA general counsel Richard Colbert says Trout's situation illustrates the problem with using statistical estimates for decisions that affect teacher pay and that her case raises concerns "over the constitutionality of such practices."

The association has proposed legislation during the current General Assembly that would prohibit TVAAS scores from being tied to teacher licensing.