"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 D)."
"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. It may therefore be advisable to bring your child to us before the age of 10 for a level assessment and then, if necessary, for tutoring, so that correction can be made in time.