NASHVILLE (AP) - The state's largest teachers' union is challenging a proposal that would tie teacher licensure to student test data.

The Tennessee Education Association held a press conference on Wednesday to voice its opposition to using the Tennessee Value-Added Assessment System, which measures how much students improve in a year.

Currently, professional teaching licenses are renewed for 10 years without regard to teacher effectiveness.

Under the proposal, a teacher's license renewal would depend on 50 percent of TVAAS data. Teacher evaluation and tenure status currently rely on 35 percent of student test data.

TEA president Gera Summerford said teachers are concerned because of possible flaws in student growth data that could cause a qualified teacher to mistakenly lose his or her license.

But Education Department spokeswoman Kelli Gauthier told The Associated Press in an email that the licensure policy includes an ap peals process to prevent loss of a license because of flawed data.

She added that teachers performing at a high level would not be affected by the proposed changes.

"However, habitually low performing teachers who receive the lowest possible score of 1 for three out of four years would not have their licenses renewed," she said.

The State Board of Education is scheduled to vote on the proposal Friday.