The subject of what books should or should not be allowed in public schools has been going on for decades, and will continue far into the future.
The question for me on this matter is "what purpose do the books serve in the various public education institutions?"
The general purpose of public schools is to provide a fundamental (basic) education centered around math, science, English, history, civics, etc. Toss in various trade related things like shop, home economics, journalism, etc. Stack in the arts and sports, too.
There are other things as well...sex education, for example.
The key is "what is appropriate for the various schools", which in general can be broken down by age. Very young children, for example, do not require the kind of in depth, detailed kind of information on sexually related topics that those approaching, or past the point of, puberty. Grade school students aren't at that stage of development biologically or intellectually. This isn't to say that such issues have absolutely no place. But discussions on this subject should be limited to what's appropriate for their general development at the time. A sixth grader, for example, is on the cusp of puberty (if not already there). A first grader, however, is not.
I can remember when I was 8 years old and my oldest brother and his wife were expecting their first. They were going to have a baby and I was going to be an uncle. I can remember asking my brother where babies come from. He gave me a straight forward answer without getting graphic: "They come from a hole between the mother's legs". It answered my question without having to go into details involving testicle deep penetrations, pretzel-like contortions, motor-boating boobs, oral foreplay, or any of the other more adult things that may have likely accompanied the actual conception event.
Who here remembers reading "To Kill a Mockingbird" and Atticus' approach to answering Scout's question "what is rape?" Scout is around 6 to 9 years of age in the story. She asked Atticus the question because she overheard someone use the term while out and about in town. "He sighed, and said rape was carnal knowledge of a female by force and without consent." He answered her question without going any deeper into the actual physical act because Scout, at her age, had absolutely no concept of what the act of sexual relations, the inherent biological emotions evoked by it, it's very intimate nature, and the biological/social consequences were to understand anything more. To be sure, she also didn't know what "carnal knowledge" meant, but she had an answer and she knew that to be forced into it was wrong. So "rape" was something "bad".
Children in grade school ages aren't generally hitting the point of hormonal development, and intellectual capacity, to deal with such sexual knowledge and expressions. So books in grade schools on the subject of where babies come from don't need to include that level of sexual knowledge behavior.
Middle school is a bit different. Children are beginning to come into puberty at this point and with puberty comes physiological and emotional changes that require further explanation. Still, getting overly graphic is not generally appropriate. Girls need to understand and come to grips with having their period. What it means biologically, what it involves and why it happens, and how to cope with it physically and socially. Boys need to understand that maybe they need to take showers every day, not only to stay clean (smelly bastards), but if only so they can take their morning p*ss without worrying about breaking their d**ks off trying to hit the toilet bowl. They need to understand why it's happening. Both need to understand that these changes will cause changes in their emotions, what those changes are trying to drive them to, and what consequences and responsibilities are involved.
High school is even more different. They already know the "bare bones" about sex, and now they're having to deal with full-fledged hormonally driven emotionally charged desires. They're learning more about their own personal sexuality and how to deal with it. It's all about "approach" in learning to deal with this. I told each of my kids that I didn't like to think of them as "teenagers", even though that's what they are by definition from 13 to 19. I like to think of them as "young adults". The term is an expression of what they really are and what their actual goal is: to transition into fully responsible adults, capable of taking care of themselves through intelligent decisions by understanding their duties, roles, and responsibilities, which includes "consequences".
Learning about sex and sexuality at this point is a vital part of transitioning from "young adult" to "mature adult". Because let's face it: Mother Nature is an absolute bitch, and as far as she's concerned the members of any species is an adult by definition as soon as they can procreate. Human society, however, has expectations and responsibilities that go beyond that and they MUST learn this.
Books are only a part of this. While I don't think "pornography" is appropriate, a level of graphic openness IS appropriate. What is "pornography"? My personal definition is that it's sexually explicit material whose primary intent is to sexually excite the individuals who read or view it. Though one may "learn" from it, it's NOT there to be "educational". That a given book or video MAY sexually excite a young adult does not automatically make it pornographic. Let's face it...a penis at that age pretty much thinks breathing the morning air is sexually explicit.
It is primarily the job of the parents to guide their children in ALL aspects of growing, including their general education. Far too many push that onto the public schools in the first place, but it's true. Parent's should always be involved in their children's growth and education, ensuring they're progressing adequately and that what they're learning is understood in context with life as they progress towards adulthood. Schools give information and basic technical understanding on many issues. Parent's ensure they're getting that and what it means to learn it.
So...strictly pornographic material in schools? No. Age appropriate material that students can learn relevant information on sex and sexuality? Yes.
The material can be there...and parents should be aware of what's there and be involved in what their own children are allowed to access. If parents don't want others to impress their own beliefs and norms on their children, then THEY need to get on the stick and be sure THEY are doing their jobs as parents. Because sure as Hell, if they don't, their children WILL get their answers from someone else.