How many different kinds of book can you name? Novels, dictionaries, atlases, phone-books and a good few more no doubt. But eventually you'll run out of words, even if you allow yourself compounds like "poetry book".
Your KS3 classes will run out even sooner. But how many other kinds of book can you think of - books for which you have no ready-made name? Books for reading in the bath, books of pressed flowers, big leather-bound tomes, and so on for much longer. Thanks to modification, you can provide every imaginable type of book with a name. Take a noun such as "book" or "tome", combine it with a modifier, and you have a new kind of book. In technical jargon, you have a hyponym of "book": "big book" is a hyponym of "book", just as "tome" is.
We've talked about modifiers before, but we've not said much about the most important modifiers, which are relative clauses. A relative clause is a sentence used as a modifier, so in effect they're home-made modifiers which you build as you need them. Young writers should certainly sharpen their relative-clause skills.
This is how relative clauses work. Say we've already written the sentence "I've bought a book". We could then write a second, separate sentence starting with a pronoun that refers back to the book: "It cost five pounds". But replacing the pronoun "it" by a relative pronoun (usually "who", "which" or "that") produces a relative clause: "which (or that) cost five pounds". This acts as a subordinate clause inside the first sentence:
"I've bought a book [which cost five pounds]". (We've bracketed the relative clause for convenience.) This much is child's play for any English speaker, but there's another rule for relative clauses: the relative pronoun starts the clause. If it's the clause's subject, it will already be in the right place (eg "which cost five pounds"), but in other cases it has to move out of its usual position up to the front of the clause. For example: "I like it" turns into "which I like": "I've bought a book [which I like]". This is quite difficult for a KS3 writer, so non-subject relative clauses are rare at this age.
It's all a question of confidence, so classroom practice may be helpful.
Here are some increasingly difficult sentences to turn into relative clauses after "I bought a book": I heard about it on the radio.
* I read a review of it in the paper.
* I think you told us we should read it.
* I shall be surprised if I like it.
* I know the person who wrote it.
Don't underestimate the difficulty of grammar, or the confidence-building impact of investigation, experimentation and practice.