Because there is still a difference between the job of a parent and the job of the school. If I want to expose my kids to that material, i can go to the public library and do it. My choice. Kids that are in that class don't have a choice on the material they have to use to pass the class not to mention the material goes way beyond a normal sex ed class review.
Like you mentioned earlier, most of these materials are vetted but that doesn't mean that there need to be adjustments when the public learns of the materials as it should be their right.