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