Prepositions in a writer's toolkit are like nails and screws in a carpenter's. They are ideal for fixing things together. For example they can fix two nouns together: "book about poetry", "class of children", "school in Manchester". The noun following the preposition in these examples is called its object and in all these examples the preposition is glued to its object. As so often with grammar, we have choices to make.
Here's an example. Which of the following do you prefer?
lWho does he work for?
lFor whom does he work?
If you're like us, the first is the only serious option in ordinary conversation, but you might write the second, with its obligatory "whom", in a rather formal style (just within reach of key stage 3 writers). In grammar terms, the choice here is between "stranded" prepositions (who ....
for) and unstranded prepositions (for whom); between separating the preposition from its object and keeping the two together.
In our experience, KS3 writers are very confident with stranded prepositions, presumably because they are so common in everyday speech. But they find unstranded prepositions confusing. We find examples like these: l"This year it is Grease in which I'm in".
lOr even: "Juliet's parents insists she marries County Paris for whom she has no interest in."
These ambitious writers have made a praiseworthy attempt to use the formal, unstranded pattern; but got confused. They bravely put the preposition at the start of the clause (in which, for whom) but then they lost their nerve and added an extra one at the end.
How can we help writers like these to break through the preposition barrier?
The main point for them to grasp is: there's actually just one preposition, and the only choice is where to put it.
One way to get this idea across is to look at simple questions, which work much like relative clauses but are grammatically simpler.
For example: "Which book did they write it in?" Where else could pupils put "in"? The alternative: "In which book did they write it?", is the only alternative - with the one preposition before its object "which book" instead of at the end.
A few examples like this will show that they can move the preposition in front of its object, the question word. Then turn them loose on relative clauses like "the book which they wrote it in" and see if they can work out how to unstrand the preposition.