In many of their school subjects, students are asked to compare ideas, themes or texts. How we say that things are similar is more complicated than we might expect. The ubiquitous word "like" doesn't always do the job. Take this sentence: "There are many professions to admire - people like teachers". The "like" there creates an ambiguity. Are we talking of teachers as a sub-set of people, or are we saying that people feel affectionate towards teachers?
The word "such" seems a better bet, as in: "I admire people such as teachers." This takes teachers as the point of comparison, and defines a larger group of people who are (in some sense) similar to them (people).
From a KS3 vantage point, the trouble is that although pupils already know how to use "like" (and often use it in everyday conversation), "such" rarely passes their (or anyone else's) lips. How can they learn how to use it?
The first problem is that it doesn't work like its synonym "like". We talk about "people like teachers", but not about "people such teachers".
Similarly, we can talk about "such people", but not about "like people".
These two synonyms belong to completely different word classes. "Like" is a fairly ordinary preposition, but "such" isn't. It's more like its other synonym, "similar" - an adjective. But it's also different; for example, you can "be similar", but you can't "be such".
As so often, the best way to teach a word like "such" is to explore a collection of examples which illustrate its various uses. You'll generally find that words like this are already lurking in the darker edges of the KS3 grammatical world, so pupils are not starting from scratch. All they're short of is details and understanding, so your role is to extend their knowledge and deepen it. You can do that by expanding their list of examples to cover all the possibilities, and systematising it to show patterns. Here's how this little lesson-chunk might go.
Suppose they provide: "such things" and "things such as this". You then offer "such things as this", showing that "such" can sit either before or after the modified noun; but maybe you'll agree that it's better to keep "such as" together. Then you change from plural to singular: "such a thing". What's odd about this? (Contrast it with: "a similar thing".) Then you add "as this", giving either: "such a thing as this" or: "a thing such as this". At the end you pull it all together with a summary of the special patterns that have emerged from the discussion:
* "a thingthings such as ..."
* "such a thingthings (as ...)" Hopefully your explanation has illuminated another little dark corner of their toolkit. And, incidentally, it's not just "such" that has these peculiarities. But that's another column.