mrz - my hypothesis for an explanation of the very poor literacy outcomes of the disadvantaged in French schools is that early literacy teaching is poor, disjointed and far too rapid: DC are taught to write (penmanship) in the second half of grande section de maternelle (Y1), to read (to decode) in CP (Y2) and to spell in CE1 (Y3). Very little reading of whole books goes on at school - hearing children read aloud and encouraging them to read silently is left to parents (or no-one). The teaching materials for early literacy are very backwards compared to the UK - both those available to schools and those available to parents.
Children from French speaking literate homes have the gaps plugged by parents, consciously or unconsciously. Children from non-French speaking and/or less literate homes accumulate weaknesses over the years - poor vocabulary, non-existent reading stamina, poor reading comprehension etc. These weaknesses don't show up hugely at the end of primary because the expected standard is low (ie the best pupils are unable to demonstrate just how much further ahead they are than the worst performing pupils) but the visible measurable gap is huge by the end of collège (Y10).