I would have to disagree with you. Aside from imparting our children with hard knowledge such as writing, reading, and arithmetic, it's always been the job of the schools to impart a strong sense of American culture to them as well. When I went to school in the 60's and 70s, we had events that reinforced each national holiday, and that's an important term: national holiday as these are all a part of a culture. We also did plays, concerts, and other events that emphasized our American heritage. Halloween isn't the local Fall Festival. It is a national event and an integral part of our culture.
I think why this case is reverberating so much is it's another attempt to de-nationalize our population. There has been a sustained effort for close to two decades now to eradicate as much American culture as possible. There have been more efforts to ban the Pledge of Allegiance than I can count. Christmas is pretty much gone. Easter is gone. Kids get suspended for wearing pro-military/veteran shirts near Memorial or Veterans Days, and in their place we're seeing more efforts to push not just personal diversity but cultural diversity.
When I went to school, we had "Pride" days where the Italian, Polish, Irish, German, etc., kids would come in dressed in the colors or clothing of their ancestral country, they would bring food their parents made that reflected these countries, and they would give talks, or their parents would give talks, about what it was like to live there or visit there. These were very informative, but the other 269 or so days of the year, these kids were expected to follow American culture, not some bland universal utopian one-size-fits-all culture where we have the post-Summer holiday followed by the Fall holiday, followed by the Winter holiday because we don't want someone with a foreign culture to feel left out.
This is America last I checked, celebrating Christmas, Easter, and Halloween have always been a part of our modern culture, and that's what our schools should be imparting to our kids. I did ten years in the Navy, 20+ years as a defense contractor, and have been just about everywhere but the South Pole and Russia, and I can tell you no other country's schools treat their own national culture this way.