"The available evidence shows that the quality of teachers is the main determinant of students' performance at school. A study conducted in Tennessee ten years ago showed that if two average eight-year-olds are placed with different teachers (one with a good teacher and one with a poor teacher), their performance can differ by more than 50 percentage points within three years (i.e., an average student can be an A or a B)."

"The negative impact of poorly performing teachers is strong, especially in the first years of school. Primary school pupils who spend several years with a poor teacher fall virtually irrecoverably behind. In some school systems, children who are in the top 20% in literacy and numeracy exams at age seven are twice as likely to earn a university degree as those in the bottom 20%."

"The data available overall shows that even in well-functioning systems, it is true that students, who do not make enough progress in the first few years of school without suitable teachers are unlikely to catch up."

Source: "McKinsey (2007): How the world’s best-performing school systems come out on top"

We would add that, because weak teachers in the lower school often cover up problems with good grades, there is a high risk that by the time parents realise it, the backlog is already there. Retrieved from can therefore be the child before the age of 10 for a level assessment and then, if required, for tutoring, so that the correction can be made in time.