Genetic performance

Identical twins have school test scores that are more closely correlated than scores from fraternal twins.

de Zeeuw et al look at data from the Netherlands Twin Register.

Even children attending the same primary school and taught by the same teacher differ greatly in their performance. In the Netherlands, performance at the end of primary school determines the enrollment in a particular level of secondary education. Identifying the impact of genes and the environment on individual differences in educational achievement between children is important. The Netherlands Twin Register has collected data on scores of tests used in primary school (ages 6 to 12) to monitor a child's educational progress in four domains, i.e. arithmetic, word reading, reading comprehension and spelling (1058 MZ and 1734 DZ twin pairs), and of a final test (2451 MZ and 4569 DZ twin pairs) in a large Dutch cohort. In general, individual differences in educational achievement were to a large extent due to genes and the influence of the family environment was negligible. Moreover, there is no evidence for gender differences in the underlying etiology.

Standard measures of trait heritability look at the variance of outcomes across identical twin pairs (monozygotic) and compares it to the variance of outcomes across fraternal twin pairs (dizygotic). Where identical twins are much closer together than fraternal twins, the trait is more heritable. Where identical and fraternal twins have more similar variances in outcomes, the trait is less heritable.

Common environmental effects refer to parts of the environment that are shared by twin pairs. So: home environment, classroom environment (where they're in the same classroom), neighbourhood and the like. Unique environmental effects are things unique to the individual and not shared with the twin.

Results?

Genetic effects were the most important contributor to individual differences in educational achievement in arithmetic (60–74%), word reading (72–82%) and reading comprehension (54–64%) and for most grades in spelling (33–70%). Common environmental effects had a negligible influence on arithmetic (0–8%), word reading (0–7%) and reading comprehension (1–12%) and a slightly larger influence on spelling (0–29%). Unique environmental effects explained the remaining variance in arithmetic (26–34%), word reading (11–29%), reading comprehension (32–35%) and spelling (30–39%). Genes were also the largest contributor to the variation in the educational achievement test (74%). The heritability differed somewhat between the educational domains measured with this test, i.e. arithmetic (68%), language (67%), study skills (60%) and science and social studies (56%). The common environmental effects were also small for the total score (8%), arithmetic (5%), language (10%), study skills (6%) and science and social studies (21%). Unique environmental effects explained the remaining variance (18–34%).

And the necessary policy caveat:

Although the individual differences between children in their achievement across different educational domains are for a large part due to innate differences, one must keep in mind that this heritability does not equal determinism. First of all, the variance between children may be heritable, but the mean can be positively influenced by a school environment of good quality.

That's true. But it's also true that if there are systematic differences in students across different schools, you can expect systematic differences in outcomes across schools.