The mathematics content in most secondary school curricula is 500 years old. In most classrooms, math lessons are based on a set of immovable facts and formulas, one separate from another. Students learn algebra and then learn geometry, as separate, unrelated operations.
Students memorize and practice a set of strategies that march along with the facts of the moment.
Then they fill in this information on test papers. And then they forget it. They forget because learning information in isolation is the least productive way to learn it. Periodically, school districts try to revamp a curriculum. Typically, they do this by handing out new textbooks.