You can compress, you can't skip- at least not in the way you're suggesting. Apart from Music 2+Ext and the Beginners languages, you can legally compress any and all of the prelim/HSC courses, *however* it is not always a good idea to do and not all institutions will allow you to do it. For instance, fresh out of year 10 you could apply to do Mathematics+Ext 1+Ext 2 and sit the HSC for it that year, but that would probably be insane to do unless you had an intense gift in math. Its also problematic in that, apart from at TAFEs you can't really compress anywhere else - schools would have difficulty teaching you because of the schedule their classes run to so most wouldn't do it...and the other option is to do it externally, but that is making it even harder on yourself again. Furthermore, people who do go right from year 10 into the compressed program usually take less subjects because it is such a heavy load - in essence it is taking 22 units or more in 3 terms, admittedly some of those units may be easier than others but that is still the constraint if you want to walk right into year 12 after year 10 with no waiting.