The full stop is probably the most used single item in any writer's toolkit. In learning about writing, pupils need to see the difference between recognising a pattern (its form) and knowing how to use it (its function). The full stop provides a good example.
Everyone recognises this tiny little dot - but that's not at all the same as knowing how to use it properly to end sentences (one function) and to signal abbreviations (a completely different function).
The form is a tool, and like other tools, there's no point in having it unless you know how to use it.
What this means for teaching the full stop is the need to be conscious of the effect you are trying to achieve. Writing should be written with the reader in mind, and punctuation is one key ingredient in helping the reader to understand what you're saying. So it's worth devoting a fair bit of time to the full stop, the most important of the punctuation marks.
Pupils can see the form of the full stop occurring as part of other marks (? ! : ; I). They need to know that each of these has its own functions, and to know how these other punctuation marks add something special of their own.
Compare the effect of three subtly different examples like these:
* The dog barked. He seemed uneasy.
* The dog barked; he seemed uneasy.
* The dog barked: he seemed uneasy.
You find these subtleties in any mature writer's toolkit, and pupils need to be equipped to use them too, actively exploring meanings and hammering home their understanding of the purposes of different punctuation marks.
That will naturally lead on to another point - the difference between usage and arbitrary "grammar rules". For example, how about full stops before "and"? At key stage 2, it's probably a good idea to discourage them so as to consolidate the basics, but KS3 writers should know that successful writers often put sentence punctuation before co-ordinating conjunctions.
It comes down, again, to the effect writers want to have, to the emphasis they are giving.
And with that flourish we bow out, hoping that our columns have helped to build a bridge between the research world of grammarians and the teaching world of schools.