Complex sentence: It would later be fiercely opposed by many parents and mathematics professionals, and rejected by many states and school districts who complained about replacing instruction in arithmetic with writing, coloring, counting, and inventing mathematics unrecognizable to any previous generation of mathematicians or educators.
Split into simpler sentences: It would later be fiercely opposed by many parents and mathematics professionals, and rejected by many states and school districts. They would complain about replacing instruction in arithmetic with writing, coloring, counting, and inventing mathematics unrecognizable to any previous generation of mathematicians or educators.