Schools are still able to select their books (book adoption)-—but just barely.
That being said, there are, indeed, writers/publishers who are redder than red.
Fortunately all my children are out of public education, even though I get to pay for the continuance of the liberal indoctrination to all those that come after. I have been year in and year out dismayed by the inconsistent method of teaching any given subject. Some years 'science' books were not allowed to leave the classroom, because they were supposedly so expensive to have each student have their own. And there was a 10 minute time allowance on 'parent's' night to glance through the textbook.
From year to year it was always something 'new' and completely out of step with what had been used the prior year. One year in middle school one of my children had three different math books, and the class was teaching to the end of the year 'state' required test.
Personally, I believe public education has been conservatives biggest misstep and the core reason why we are where we are today. The powers that be have gone along to get along ignoring what goes for education in K-12. And these public school teachers and administrators/counselors do not like to be questioned.
Rush describes this nation as being 'bi-polar', well imho, it was done by design via public education using what is commonly called the scientific methodology.