Tennessee Schools Cooperative purchases filtering systems from Education Networks of America (ENA), a Nashville-based technology company that serves schools, libraries and government agencies.
The company can block a variety of categories, including sites related to abortion, alcohol and pornography. On their Web site, the category "LGBT" is open to all schools, except those like Tennessee and Indiana, which are asterisked.
That category includes Web sites that "provide information regarding, support, promote, or cater to one's sexual orientation or gender identity including but not limited to lesbian, gay, bi-sexual, and transgender sites."
"This category does not include sites that are sexually gratuitous in nature which would typically fall under the pornography category," according to the ENA Web site.
Some of the blocked sites are "well-respected" organizations, according to the ACLU, such as the Human Rights Campaign; Parents, Family and Friends of Lesbians and Gays, The Gay Straight Alliance; and the Gay-Lesbian Straight Education Network.
The ACLU says the category, which is separate from a pornography grouping, singles out and blocks "protected speech."
"Moreover," states its letter, "the filtering scheme engages in impermissible viewpoint discrimination" by blocking sites that are accepting and tolerant of gays.
The organization said this filter also violates the Equal Access Act because it prevents student groups like gay-straight alliances to "school resources and privileges" afforded other non-curricular clubs.
In 2001, Congress enacted the Children's Internet Protection Act to address concerns about access to "offensive content" over the Internet on school and library computers.
Since the, companies like ENA have provided filtering solutions with a database of categories from the company Blue Coat. School districts that contract with ENA decide which of those groups are appropriate for their students.
"We do not make those decisions, our customers do," said ENA President David N. Pierce.
ENA provides schools with an "authorized override" which gives certain teachers a password to access Web sites in a blocked category, "for a specific class or period of time or a research project," he told ABCNews.com
School districts can also request that a site within a category be reconsidered as appropriate and be permanently unblocked, he said.
"I expect Tennessee will meet on an emergency basis in the next few days and if they believe as a school system that we should unblock them, we will do what they tell us to do," said Pierce.
This is not the first time the ACLU has fought to stop filtering of gay Web sites. In Palm Beach County, Florida, in 2006, a high school teacher who was researching violence and discrimination against LGBT students, couldn't get access to GLSEN's site.
"The teacher was surprised that it was blocked," said Rand Hoch, president of the Palm Beach County Human Rights Council, who worked with the ACLU to convince school officials that students deserved access.
Oddly, sites that used terms like "homosexual," instead of "gay" or "lesbian," were deemed appropriate, said Hoch. "Those sites condemned homosexuality."
"His students were uncomfortable enough and GLSEN was a good site to help them." he said. "It wasn't the least bit pornographic."