If you were able to get into JMSS then I'm sure the same procedures apply for the Mac.Rob exam however I would believe it to be more application based (so having the ability to show evidence of something) which will greatly play a part even though there's the exam component. Personally, I believe English is something that should be your strong point when going into the exam (rather than Maths) since English is a compulsory subject (but let's just leave it at that
)
Though I do have to ask... if you're going to be in Year 10 at JMSS next year then why don't you wait and go experience JMSS before making the decision to go to Mac.Rob (since JMSS is a pretty good school nonetheless)!
(I actually find it weird that Mac.Rob has an entrance exam for Year 11 entry compared to MHS where it's purely application/interview based and no exam).