Don't give her false hopes; oh I got an E in methods and it went up to an A....oh yh...but y? cuz u owned yur VCAA exam idiot.
SACS at MHS/Macrob count for less relative to other schools bcoz the cohort lifts yu' up as they smash the exams.
And yh trust me the competitive atmosphere will get 2u unless o' course yur a super uber nerd anyway 
You're in year 10, what makes you think you know the VCE system better than him, let alone call him an idiot?
You are wrong, your own exam mark will not lift up your own sac mark with exception to VERY rare cases. It works on the principle that even if you get 0 marks on your sacs all year, if your whole class gets an A+ on their exam, then you will get A+ A+ A+ on your end of year VCE result. But if you are last in your cohort and you get an A+ on your exam, and the worst exam mark is a D+, you're stuck with that D+ for your SAC mark irregardless.
The competitive atmosphere doesn't "get to you" either, it's just a facade created by students who try to be super competitive.
Let's just say you can afford to make a slip up, and it is easier to get a 36-42 SS than it is at most other schools, but you will still have to work hard to get anything above that, just like anyone else would 
so lyk i said...the cohort lifts yu' up and thus the SACS count for less relative to other schools where you have to maintain that rank 1 as you fear your cohort will bring you down as you illustrated with yur D+ example...yu wudn't wna b last in a shit school, yu wudn't wna b last anywher but being last in MacRob/MHS is better then anywhere else.
Yep, what taiga said is pretty much what happens. My friend lost 1 mark on the English exam - which is normally 50 material - but he was ranked near the bottom in internal SACs due to him not bothering to read the texts and the teacher hating him (which was justified really). In the end, he ended up getting Ds and Es for his actual SAC mark and got under 30 SS . This is quite an extreme case, but it shows the point that being in a selective school only provides a slight buffer such that if you fall a bit short on the exam, you'll probably get your marks back. It's quite unlikely to effectively boost your score above what it should have been. People only think this because they're surrounded by people smarter than them, so they think that their internal A getting scaled to an A+ was amazing. Well, you probably would've gotten that A+ internally at another school anyway. So yes, the cohort only really acts as a buffer for slight underperformance in the exam I guess.
What's important is the atmosphere and academic support you get instead. I was going to say facilities as well, but McRob fails in that regard (MHS on the other hand has great facilities). But yeh, people often get pressured at other schools to not study or even try at all in VCE whereas people are more understanding in MHS/McRob if you have to pass up on something in order to study. And as others have pointed out, being in a competitive environment makes you try harder. It feels almost as if people have been framing this as a negative thing but it's actually good. For the most part, I guess this competitive environment is more of an internal thing than external - that is, seeing everyone else working so hard and achieving such high scores makes you work harder yourself. This happened to me when I got a sudden 'oh shit' feeling after reading my friend's essays in year 12 and suddenly had a burst of motivation to start gunning English since I realised how far behind I was. It's not a competitive environment in the sense that everyone compares their marks to each other and then bags out those who scored lowly. People are generally very supportive there.