Another modern problem with education is that every student has a smartphone now, which they bring to the class, then they get distracted by it.
Maybe more and more teachers care less and less about training students to take tests, and care more about actually educating students in the subject matter. So maybe the plunging test scores are are simple as “students are worse at taking tests.”
"Modern" education isn't modern at all. It's a leftover from industrialization, aiming at producing easily controllable worker drones.
"Elites" fearing the "plebs" to question them on things they have no good answers for is what constrains active development of actual modern scientific concepts of pedagogy taking hold. You only get to research what is being paid for.
Look at the, pretty obvious, natural human child-mode of learning: playing games.
Just not escapist nonsense games. Games that model relevant aspects of reality in an engaging way. Where you can fail without real-life consequences and learn from it while enjoying yourself.
This is highly optional. You can lock phones up at the start of the school day or at the start of class