State school graduates do better at university than private school graduates with the same end-of-school tertiary entrance score. That’s the clear finding in a number of Australian studies since the 1980s (here, here, here and here), and in England since the 1990s (here, here, and here).
The Australian research compared academic results at the end of first year at particular universities for cohorts whose entry was based on tertiary entrance scores (now ATAR) for the previous year in the same state.
The differences between graduates of state and private schools were substantial (though less pronounced among those who did very well at university). The Australian research found that, on average, graduates of state schools received the same marks at the end of first-year university as graduates of private schools who had tertiary entrance scores around three to six points higher.
– Barbara Preston