The French Community of Belgium scored around the OECD average in all three subjects in the latest PISA study, but its French-speaking education system still lags well behind the Dutch-speaking education system.
The PISA assessment, conducted every three years, measures the performance of 15-year-old pupils in reading, mathematics and science across 91 participating countries.
Both Flanders and the French Community have seen standards decline over the past 15 years. Even so, Flanders still ranks among the top 20 countries, while the French Community places about 10 positions lower.
The latest study also found that inequality among pupils in French-speaking education has fallen. The system was long seen as one of the most unequal in the world, meaning it did less than most to act as a social ladder.
That appears to be less true now, although the system still performs below average on that measure.
Researchers said part of the gap can be explained by a socio-economic context that is less favourable than the international average. In particular, the share of children with a migrant background is higher.
Twenty years ago, Flemish education ranked among the best, but the region has been slipping year after year. The number of pupils failing to master basic skills is rising, while the share of top performers continues to shrink.
Thirty per cent of pupils now fail to reach the basic level in reading.
The fall in maths has also been speeding up. Until 2013, there were more top performers than low performers, but that trend has since reversed. A quarter of pupils no longer reach the basic level in maths.
Although Flanders and the French community in Belgium are dropping in the rankings, the fall in standards is visible across Europe.
