Your question is an important one because no one knows the answers to that and many other questions about the learning and teaching of punctuation.
Everybody has opinions about punctuation but there is precious little evidence to support any of them. Compared with other areas of English education for which there are vast amounts of research (spelling, for instance), there is virtually no research about how children learn to punctuate and how best to teach punctuation.
It seems that the stipulations of the National Curriculum are not based upon evidence.
The Punctuation Project
School of Education
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