By Kiah Collier A statewide teachers group filed a lawsuit Wednesday in an attempt to block the state from implementing a controversial system that for the first time ties assessments of educators to student performance on standardized tests. In a lawsuit filed against Texas Education Commissioner Mike Morath in Travis County District Court, the Texas State Teachers Association alleges that the new teacher evaluation system — the Texas Teacher Evaluation and Support System, or T-TESS — violates state law by requiring school districts to base 20 percent of each teacher's evaluation on “student growth measures” that include standardized test scores. Those student growth measures may include “value added measures,” or VAM, which are "typically based on a complicated formula that compares actual student test scores to the scores predicted by a mathematical target based on the standardized test scores of similar student populations,” the association explained in a statement. “TSTA contends that state law ... clearly requires a teacher appraisal system adopted by the commissioner to be based on ‘observable, job-related behavior,’” the statement said. “But a VAM model is not ‘observable’ and is not even available to teachers and others who wish to understand the basis for their evaluations.” “Teachers are not robots, and their performance should be evaluated by an easily understood, transparent system that helps them perfect their job performance,” association President Noel Candelaria said. "Educators’ compensation and jobs are potentially on the line here, and their work must be evaluated fairly – and legally." The new teacher evaluation system, which the state has been piloting for over a year, is set to take effect July 1. Participating school districts will use it to make pay, employment and other consequential decisions. It replaces a nearly 20-year-old state-recommended teacher evaluation method known as Professional Development and Appraisal System, or PDAS. School districts are not required to use the state-recommended evaluation system, although most do to ensure compliance with state and federal regulations. Texas Education Agency spokeswoman Debbie Ratcliffe said Wednesday that neither Morath nor the agency could comment on the lawsuit because they had not yet been served with it. The state law the teachers association cites in its lawsuit says that the education commissioner shall adopt a recommended appraisal process and criteria on which to appraise the performance of teachers and says that the criteria "must be based on observable, job-related behavior,” including “the performance of teachers’ students.” The rules Morath approved this month to implement T-TESS clarified the phrase "performance of teachers’ students" to mean "student growth at the individual teacher level,” according to background information the agency posted online that says teachers should be assessed based on their students' academic progress rather than their outright proficiency. Morath, who assumed his role as head of the Texas Education Agency in January, created four “broad” ways to measure student growth for individual teachers, including student portfolios and scores on both statewide and district-level exams. Morath’s predecessor, Michael Williams, also supported the new teacher evaluation system but refused to require all school districts to use the same assessment method — even when the state was at risk of losing a waiver from the federal No Child Left Behind Act. The association noted in a statement that even for school districts that create their own appraisal system, state law “sets the same ‘observable, job-related behavior’ requirement." The business-backed Texas Institute for Education Reform blasted the lawsuit Wednesday as "an affront to teacher quality because T-TESS is a significant step forward for Texas teachers.” "It replaces a system that rated virtually all teachers’ performance the same and provides almost no useable feedback,” said Courtney Boswell, the group’s executive director. "Instead, T-TESS identifies high-performing teachers as well as low-performing ones with a focus on moving all teachers toward excellence." Disclosure: The Texas State Teachers Association and the Texas Institute for Education Reform are corporate sponsors of The Texas Tribune. A complete list of Tribune donors and sponsors can be viewed here.