The actual work you do in prelim and the results you get in it have absolutely no contribution to the actual HSC, however in a sense the work you learn from prelim does help for most (not all) subjects and in other subjects it is essential to HSC. For example in english, prelim doesnt count at all to HSC but it gets you used to essay structuring and writing analytical paragraphs, mathematics (which may have been what your teacher was talking about) gets carried over to HSC with up to 30% of the HSC mathematics exams being prelim iirc, sciences dont contribute to HSC however it would still be a pretty good idea to learn your prelim content obviously, other subjects like social sciences i dont think carry over to the HSC at all but generally it would be a pretty good idea to make sure you do well in your prelim as that can act as a base for which you can improve on when you get to year 12, just dont overstress about it.
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