I will be blunt.
You will die.
Don't do it.
What you really have to consider is that basically, at least at my school, we'd learnt all the grammar by the end of year 10; year 11 was all about building familiarity with actual texts, and as such 3/4 Latin assumes that familiarity. The texts you study are by no means easy without being used to the language - poetry, in any second language, is always going to be really tough. To approach Vergil having just learnt your conjugations and barely knowing what the subjunctive is? Madness. Even the prose is not comprised of simple, straightford sentences - and may God save you if you got raw Livy as your exam unseen.
In short, if you were to pick up Latin in year 12 having never done it before, not only would you firstly have to learn all the grammar most students in the state took four years to learn (years 7-10)*, but you would lack the familiarity of the language and (most likely) the solid vocabulary required to do well in the subject.
*though admittedly, if you're good with languages, a year 12 brain would probably be able to cover most if not all of the grammar in the preceding summer holidays, but it probably wouldn't become ingrained without some intensive usage, in which case it becomes an issue of "do you really have that much time and motivation to do so?"