The problem starts with children not learning "enough." The first question is, "Why?"
Fortunately, Smart people have studied these issues and found that the usual culprits are poverty, homelessness, food insecurity, and family stress (divorce, parents unhappy, etc.). Kids do better when they get breakfast, lunch, and an afternoon snack/meal in school and have a safe place after school for studying and socializing. Teachers do better with enough money to eliminate their financial stresses and total funding for classroom basics such as properly maintained buildings, heating and cooling, and classroom supplies.
But instead of addressing these issues, some people shake their fingers at the teachers, proclaiming the teachers union is the problem.