It is not explicitly in the HSC syllabus. In other words an exam can never test comparative advantage directly, although sometimes exams are not written to the syllabus which can make it difficult to be certain.
It is however quite an important concept in economics. It is essentially the justification of the free market. Not just trade amongst nations, but trade amongst economic agents. Adam Smith began with a discussion on specialisation, and David Ricardo formalised the notion of comparative advantage. This is pretty important stuff and I think any student whether it appears in the syllabus or not, should know what it is about.
It can be used in an essay as well. For example you can discuss the merits of trade with respect to comparative advantage when talking about protectionism.