How do you prove merit? Test scores do not only reflect the work put in by teachers. The best teacher in the entire world would struggle to get kids in Baltimore City to pass standardized tests, and a crappy teacher could get a wealthier school in the county to pass with flying colors. Proving quality teaching is next to impossible to do, outside of basic observational evaluation by school administrators.
Teachers are the educational system's workhorses, they put in all the work. But they do not choose the students, they do not choose the parents, they do not choose the curriculum, they do not choose the rules, etc. So I do not think it's fair to place a heavy blame teachers for America's underperforming students. There are too many other factors.