I agree with you that "bad teachers" is not the core or only issue. A more serious issue, as an example, is that good teachers are actively driven away by petty bureaucrats. Those that remain are severely constrained in the good they can do. With the advent of endemic standardized testing tied into bonuses and tenure, classes have devolved into regurgitate the answer mode. That issue, of really excellent teaching being punished or prohibited, is only one of many many problems in the system. Easy fixes don't work because there is not just one thing that is the problem. There are many things and the things vary somewhat state by state and district by district within a state.