This year's national curriculum assessment results reveal that boys' performance has worsened at key stages 1 and 3 compared to their female classmates.
More girls achieved the core subject indicator (CSI) - the expected level of attainment in English, maths, science and first-language Welsh - than boys in all key stages again this year.
Last year at KS1 boys managed to close the achievement gap with girls from 8.4 percentage points to 8 points, but this year the gap widened to 8.7 points. At KS3 the gap has grown to more than 10 percentage points for the first time.
Although boys have managed to narrow the achievement gap at KS2 from 9 to 7.8 percentage points, a question mark hangs over those results after the reliability of the assessments were called into question by Welsh inspectorate Estyn this year. Inspectors said the lack of a national system for verifying KS2 accuracy is causing concern.
In a separate report, Estyn said it was also concerned that pupils' progress in KS3 is being hampered by schools focusing their attention on post-14 curriculum developments.
David Egan, professor of education at the University of Wales Institute Cardiff, said: "Although boys results are improving, girls are still improving faster, and that's worrying. The biggest problem is poor literacy levels among working-class boys, and that's something we have got to get to grips with."
A spokeswoman for the Assembly government said: "The results this year show positive progress across the board, building on last year's success across the core subjects at each key stage.
"Of course, we cannot afford to be complacent - we must be realistic about the need for further improvements in performance, without undermining the very real progress made to date."
- Original headline: Boys lag behind girls at every educational stage . again