Just a warning - don't let people tell you drama is a bludge. IT'S NOT. Okay, it is in year eleven, but in year twelve it's actually a heck of a lot of work - two major works PLUS theory (essay) work as well! It easily takes up the most time of all my (admittedly rather easy) subjects.
Thing is, it's a heck of a lot of fun, and if you want to be a teacher, I think it'd be a lot more use to you than Legal or Ancient. See, drama improves your confidence speaking in front of a group of people - I know that in practice exercises I did a few years ago where we had to "teach" the class for a lesson I froze up, stuttered, didn't know what to say, got stage fright. But now after doing drama I'm fine with being told to "take the class" (happened a few months ago when I'd been to see the play we were studying and the teacher hadn't, so he figured I'd be more qualified haha), can jump up and do it fear-free on total improv. Drama has cured me of nervousness of public speaking. And I'm a better liar too! But then again, every I know who does Legal loves it, apparently it's really interesting and relevant to real life.
As for Ancient, I love it because it's such a low-pressure subject (that could just be the teacher, though). No need to memorise dates because few historians are really sure of the dates anyway, there's a lot of reading involved and you have to write fairly extensive summaries but it's all pretty simple and straightforward.
Don't be scared of doing thirteen units! I'm doing fourteen and loving it, I didn't wanna drop any of them so I simply didn't
It is a bit stressful though come Trials though, sticking to twelve is probably the
smart choice...
Oh yeah - Earth - again, a very simple, straightforward subject. Year eleven is boring as all hell, just rocks, rocks, water, some more rocks, but year twelve is fun - evolution, volcanoes, fossils, field trips.
But I'd still recommend switching Legal with Drama. Just sayin'. I did 2unit maths in year eleven and hated it to death, so I dropped it. Honestly, unless you like it or want to do a science/maths/engineering/medicine at uni I don't see the point in doing it... but each to their own.
Good luck!