Updated at 9:35 p.m. Tuesday: Revised to show the school board voted to keep the poster off the classroom door.

A fight over whether to allow a poster of A Charlie Brown Christmas at a Texas school has escalated, with the attorney general and a religious nonprofit pressing Killeen ISD to reverse a ban on an employee's religious display.

The school board on Tuesday night voted 6-1 to keep the holiday decoration off the classroom door, KWTX-TV reported.

District officials have defended the decision by explaining that employees can't impose their personal beliefs on students.

Texas Attorney General Ken Paxton chided the district last week after a middle school principal asked clinic aide Dedra Shannon to take down part of a 6-foot poster on her door. The poster was based on A Charlie Brown Christmas — the classic TV special about childhood innocence and the true meaning of the holidays — and showed the character Linus quoting a Bible verse.

A picture of Dedra Shannon's poster in a letter sent by a nonprofit to Killeen ISD. ((Texas Values))

On Tuesday, Paxton pushed again. He wrote a letter to the school board urging it to rescind "this unlawful policy" while demanding an apology to Shannon.

"Based on public comments, both the principal and Killeen ISD objected to the Christmas poster for fear that it violated the First Amendment and would lead to a lawsuit," Paxton wrote. "Although these concerns are not surprising in an age of frivolous litigation by anti-Christian interest groups, they stem from an incorrect reading of the law."

AG Paxton Releases Letter Sent to Killeen ISD Board of Trustees onForced removal of a Christmas decoration pic.twitter.com/HzcfQvTwaf — Texas State News (@TexasStateNew) December 13, 2016

Shannon, the clinic aide, has secured legal representation from the religious nonprofit Texas Values, which wrote its own letter to the school district. Attorney Jonathan Saenz said there was nothing illegal about Shannon's poster, noting that it attributed a quote to Linus and not the Bible.

"No one is 'imposing' their beliefs on anyone simply by decorating a door, any more than someone who decorates their house with a Nativity scene and a Bible verse is 'imposing' their beliefs on their neighbors," Saenz wrote. "Ms. Shannon has not threatened or coerced any student who disagrees with her beliefs."

The First Amendment forbids the government from establishing an official religion. But Saenz told district officials that the poster was no more an "establishment of religion" than the Pledge of Allegiance, which contains the phrase "one nation under God" and which children recite in school.

Paxton warned last week that Killeen ISD's decision was "an attack on religious liberty and a violation of the First Amendment and state law."

He was referring to the so-called Merry Christmas Law that Texas legislators passed in 2013. It removes legal risks from exchanging holiday greetings in classrooms and protects symbols such as Christmas trees, menorahs or Nativity scenes — that is, as long as more than one religion is represented and a secular symbol such as a snowman is displayed.

But Killeen ISD officials told KWTX-TV that the law doesn't protect the poster because it requires that a display not encourage adherence to a particular religion.

The poster cites a passage from the Gospel of Luke spoken by Charlies Brown's best friend in the TV special.

"For unto you is born this day in the city of David a Savior, which is Christ the Lord. ... That's what Christmas is all about, Charlie Brown," the poster reads.

The Merry Christmas Law was briefly summoned in North Texas three years ago. A Frisco school drew criticism after a parent volunteer emailed others to say that they had to stay clear from Christmas references and the colors green and red in planning for winter parties.

The matter was quickly resolved. Frisco ISD officials said at the time that there was no ban on Christmas references and that the parent's email had been taken out of context.

Staff writer Claire Z. Cardona contributed to this report.