The great state of Texas is famous for its cowboys, its Southern hospitality, its oil - and, within the United States, for the power it wields over the country's school textbooks.

Because of its size, Texas is the country's second-largest purchaser of textbooks - meaning that publishers, ever in pursuit of the largest possible markets, are under pressure to ensure they win approval from Texas State Board of Education (SBOE).

But this body has long been criticised as partisan, with SBOE members accused of trying to stamp their political and religious views on the school curriculum.

Dan Quinn, communications director at the Texas Freedom Network, an advocacy group that supports religious freedom, told Al Jazeera that the SBOE was bringing a far-right bias to Texas classrooms. "Social conservatives began a concerted effort to take control of the state board of education in the early 1990s," he said. "As that faction's power on the board has increased, so has its influence over the process of adopting state curriculum standards and textbooks that must be based on those standards."

He added: "The board's far-right bloc rejects separation of church and state, opposes teaching students responsible sex education, demands that biology textbooks include creationist arguments against evolution, dismisses concerns about climate change and other environmental problems, and promotes fear and ignorance regarding religious minorities in this country."

The teaching of evolution is perhaps the biggest lightning rod for debate. A recent discussion over which books should be used resulted in a delay in the approval of certain biology textbooks because of one board member's concerns with their treatment of evolution.

READ MORE: The great Texas textbook debate