The policy, which dates at least to 2006, covers the approximately 700 students who are accepted into the state-funded Delhi Charter School in Delhi, La.

“There have never been any complaints from students or parents about the school policy,” Chris Broussard, the school’s teacher-director for grades 6-12, told TODAY.com. “However, in light of the recent inquiry, the current policy has been forwarded to the law firm of Davenport, Files & Kelly … to ensure that necessary revisions are made so that our school is in full compliance with constitutional law.”

The law firm did not return a call to TODAY.com.

The American Civil Liberties Union of Louisiana sent the school a letter on Monday, saying that the policy violates federal law and the U.S. Constitution, and threatening legal action if it is not revised immediately.

“The policy discriminates against female students not just for being pregnant but even for the possibility that they might be pregnant, and treats them as though pregnancy was some kind of contagious disease by telling them they can’t stay in school,” Marjorie Esman, executive director of the American Civil Liberties Union of Louisiana, told TODAY.com. “That is a gross violation of the law and their right to have an education.”

The Christina School District had barred Brianna Moore from classes at Shue-Medill Middle School near Newark last week because, officials said, her hair violated the school’s rules against unnatural and “excessive” colors.

School policy, according to its website, allows only “natural color, brown, blond, black, natural red/auburn.”

The change of heart was noted in an e-mail from the attorney for the school district to the ACLU of Delaware, which had taken up the case.

“We’re on our way back to school now,” said her father, Kevin Moore. “That was our whole point.”

Moore said he had allowed his daughter to dye her hair pink as an incentive for improving her grades, which she did.

According to the ACLU’s version of events, the girl had moved and entered a new school as a 6th-grade student in the fall of 2010. In early 2011, she felt targeted by a school monitor and posted an update to her friends-only Facebook wall saying she “hated” the monitor because “she was mean to me,” using her own computer and while off campus.

Soon after, she was called into the principal’s office — he had obtained a screen shot of the post — and given detention.

The student subsequently posted another update to her page related to the incident: “I want to know who the f%$# told on me,” the complaint says. Again, she was called to the principal’s office, and this time was suspended for “insubordination” and banned from a class ski trip.

In March, the student had a second run-in with school authorities.  The parent of another student had complained that the girl was talking about sex with that student.  The 12-year-old was called out of class by a school counselor and eventually brought into a room with several school officials and the sheriff’s deputy, where the password demands began.

The ACLU claims that the school never asked the girl’s parents for permission to examine her private Facebook space. The school district doesn’t dispute that it obtained the girl’s password, but does say it had parental permission.