Speaking at the Wales Education conference this week, Andy Hawkins, an Estyn secondary inspector, said that Welsh schools needed to address under-achievement at key stage 3.
She said: "Joint curriculum planning is needed, so that work in Years 6 and 7 fits together, just as it does between Years 7 and 8.
"Primary and secondary teachers should be looking at work in each other's classrooms.
They should be jointly assessing the work of pupils, starting at the beginning of Year 6 and continuing into Year 7."
Estyn will publish a report this summer, recommending methods of reducing the gap between the two key stages.
Karl Davies, Welsh director of the National Association of Headteachers, said: "The principle is sound, but it would be an awful lot of extra work.
This must be resourced properly. Otherwise the benefits to teachers in making transition less burdensome would be outweighed by the extra work."
Last year, only 62.5 per cent of Welsh pupils reached the expected level five in English at KS3. Seventy-one per cent achieved this level in Welsh, 64 per cent in maths and 66.5 per cent in science.
This was well below the Welsh Assembly target of 70 per cent in each subject.
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