NASHVILLE (AP) — A parent and grandparents of three children who go to an online school that has been ordered closed because of low academic performance are suing Tennessee's education commissioner to keep it open.

The suit, filed Thursday in Davidson County Chancery Court, says the Department of Education violated state law when it ordered the Tennessee Virtual Academy to close at the end of the current school year unless it dramatically improves. The lawsuit specifically names Education Commissioner Candice McQueen.

Officials with the state Department of Education did not immediately respond to an email requesting comment.

The suit was filed by Regina Taylor of Hendersonville, on behalf of her 10-year-old twin sons Brandon and Jordan and by Dick and Patti Posan, who live in Sewanee, Tennessee. The Posans are the legal guardian of their grandson, Austin, a 13-year-old who has been diagnosed with autism, an inherited form of intellectual impairment known as Fragile X, and attention deficit hyperactivity disorder.

"He came to the Tennessee Virtual Academy not reading and now he's reading at a fourth-grade level in two years," Patti Posan said. "Oh, my God, he's a different child."

The Posans say they worry about Austin being bullied if he has to go back to a traditional public school. Taylor says her son Jordan is too medically fragile to attend a traditional school.

At the Tennessee Virtual Academy, kids stay home and do schoolwork on their computers. The Union County School Board contracts with K-12 Inc., a Virginia-based for-profit corporation, to operate the school, but the students can be located anywhere in the state.

The school has been consistently ranked at the bottom of Tennessee schools since a 2011 law created it. Critics have called it a failure and a drain on the taxpayers.

The state currently pays about $5,462 per student at the school. There are currently about 1,300 students enrolled, but that's low because the state ordered the school to stop enrolling.

Under state law, the state education commissioner has the authority to close the school if it ranks among the worst performers for three consecutive years. Last year, former Education Commissioner Kevin Huffman ordered the school to stop enrolling new students and said it would be shut down at the end of the school year unless it improved significantly. The school has consistently been ranked 1 on a 5-point scale, with 1 being the worst and 5 the best. Huffman said the school needed to improve to a 3. McQueen, who replaced Huffman, stood by the decision.

The lawsuit argues that the school actually has more time because the law giving it three years to be ranked at the bottom didn't pass until 2013. It also says the commissioner exceeded her authority by ordering the school to improve to a 3.

Taylor and the Posans are asking the judge to issue an order barring McQueen from closing the school and to find that she exceeded her authority. They also want the judge to let the school begin the enrollment process for next year. If not, they say they have a right to a hearing.