I can see how if it's Extension or Background speakers (you're actually competent and semi to completely fluent in the language), you could pick it up, but we're talking about Italian Beginners here. This means that you are not really experienced with this language in any way (perhaps just the real basics). By missing out on the entire Preliminary Course, how exactly do you intend to catch up on missed material, as well as managing the material being taught in the HSC course. Also, think of your ranking. These are people who have had about 3 terms of Preliminary Italian Beginners, they are going o be o much more advantaged than you. Sure, you can work hard and achieve excellent results in this course, but in the end, is it really worth it? You'll basically be attempting to complete the HSC course, whilst revising/ catching up on missed Preliminary content. In the end, the amount of time you may need to sacrifice for Italian Beginners in order to compensate with having missed so much of the course may impact on your performance in other subjects.