After the application of hierarchical linear models to the PISA 2015 test data to isolate the part relating to the average performance of schools not attributable to the effect of these contextual variables and obtain an average residual per school (what we understand in the project as school effectiveness), the following significant variables were found.

Group Variable Effects
Pupils Gender This is one of the most influential variables: While boys obtain higher results than girls in mathematics and science, in terms of reading comprehension, girls achieve a higher average level of performance.
Family socio-cultural level Pupils who have a greater variety of cultural and digital resources in their homes and whose parents have higher levels of education and jobs associated with a higher academic/economic status achieve higher average performance in the 3 areas.
Repeaters Repetition of an academic year is a significant factor in the three areas assessed, with repeating pupils showing lower levels of performance.
Number of school changes The number of school changes that pupils experience throughout their academic careers has an inverse effect on school performance: the greater the number of changes, the lower the level of performance in the 3 areas.
Migratory status While 1st-generation immigrant pupils are handicapped compared to natives in terms of performance, these differences dissipate with 2nd-generation immigrant pupils.
Schools Socio-cultural level of the school It seems that the average socio-economic level of the families whose children attend the school is the most influential variable at school level, significant in the 3 areas. Schools whose pupils come from families with a higher socio-cultural level achieve higher average performance.
School size In the case of Science, the size of the school has an inverse effect: Larger schools tend to show slightly lower average performance in Science subjects.
Repeater rate The repeater rate in secondary Year 4 of the school has a significant influence on performance in Science and Reading Comprehension. Classrooms in which there is a higher rate of repeaters show lower performance.
Percentage of female pupils The percentage of female pupils in a school appears to have a positive effect on performance, not only in Reading Comprehension, but also Science.