Common sense idea: if pupils don't "pass" Key Stage 2 then they should stay back another year (or until they do). This will ensure that every child starts Secondary School with a fair chance of success.
What would actually happen?
It's been tried in both the US and Canada - so some grounds to make reasonable predictions.
It won't work. First of all it makes schools amplify the already regrettable trend of just "teaching to the tests" - in British Columbia (Canada) this meant that during the year that tests took place, the children would often be doing work at a lower level than previous years (in terms of critical thinking, hand-on experiments, exploring subjects in depth). As the parent of a child coming to the end of primary school, I've seen this at first hand. For example, the constant revising of a scientific idea about forces through the endless replaying by video of a toy car rolling down a slope until everyone "got it". Great education.
Secondly, testing regimes like this create "failures at eleven" - once children failed to move on from primary school, their self-confidence drops and they would be likely to become more disaffected and less likely to learn than before.
The effects of repeated failure are unlikely to be good. It's also unlikely that primary schools want big cohorts of early-adolescent children aged 11+, feeling disaffected, on their rolls, knocking around the playground with 3 and 4 year olds.
Other things that Mr Cameron does not seem to have thought about.
Children who are not good at taking tests (don't forget, we are talking about 11 year olds here) but who may have perfectly good basic maths and English in day to day classroom life. Do they have to stay back?
Children with learning difficulties who may never achieve level 4 across the board. Do they stay in primary school until they are 16?
Schools which are genuinely not doing a good enough job - would they end up with half of their year 6 repeating? In which case, could they do a better job with a class of 45 than they could with a class of 30? Is this the best way to lead improvement?
What would happen to the unused spaces in secondary schools which result from children being kept back? As schools are funded per child on roll, a secondary school could end up being penalised financially for the perceived failures of the primary school down the road.
There is an important debate to be held about many of the issues David Cameron identified in Sunday Telegraph earlier in the week. A debate: not the floating and then spinning headline-happy ideas like repeating year 6.
Cameron hasn't looked at the findings of the "no child left behind" movement in the US. Nor are there any signs that he has given much thought to the realities of trying to run a primary school in England.
One last thought: shouldn't we concentrate less on getting children ready for secondary schooling, and more on getting secondary schools ready for children? How about offering a creative and stimulating curriculum which would interest less academic and test-savvy children (which would still include the basics of literacy and numeracy, perhaps through more practical application)? Aged 11, most children would surely do much better with that. But perhaps we should try to "fix broken Britain" by labeling them as failures and leaving them to watch their friends go off to secondary school whilst they stay behind, cross-legged, in primary school halls.