Put paragraphs in their place
Start by asking pupils what room they have just come from and where they are going next. When asked why they moved, they explain that different subjects have different rooms; that you need the gym, for example, for PE.
Once you start describing paragraphs as "rooms", you're giving them a good model of paragraphing. As they move onto a new "subject", an aspect of a topic, or a stage of a story, they need to move the reader on to a new "room". Starting a new line is like the walk between rooms, a natural pause or breather before the next stage. You can bring in the lesson bell if you wish as the signal that a new subject is starting. Get them to use a linking word, such as "in addition" or "however" as their own lesson "bell" for the reader.
In fact, the school day itself can become a familiar frame for the whole piece of writing. Their introduction can be the "registration" or "assembly" that begins the day with a clear and (hopefully) interesting look ahead. Different paragraphs need not be exactly the same length. A shorter one for effect can be a "break" between more difficult stages; a longer one might be a "double" that deals with an important point in detail. But no one wants a "triple" or "quadruple" lesson, so they need to remember to keep the reader moving on to new paragraphs.
Wouldn't it be nice if the last lesson of every day was like the ideal concluding paragraph - shorter than all the others, and leaving a final, positive impression of the "day" you have just had?
John Gallagher = Head of English, St Augustine's Catholic High School, Redditch, Worcestershire