you saying they skipped 2 grades
Yeah, started early and skipped. They are very smart though.
WOOOW...
ok then
what if:
lets say, she got about the same marks on the test as we yr9 got.. would that mean that we've got a better chance of being chosen to the interviewing process than her - because she is in a higher year..... 
(not sure if I put this to word right...)
The guidelines are here: http://www.jmss.vic.edu.au/images/JMSS_Enrolment_Policy_Year10_2016V1.pdf
To enrol at John Monash Science School in Year 10, students must have completed Year 9 in a current registered school, and turn 15 years of age during the first year of enrolment. If a student has been accelerated in their current school and their performance on the Entry Assessment is commensurate with 1.4 above, they may be admitted provided that they will turn 14 years of age by April 30th during their first year at John Monash Science School. Accelerated students will also be required to provide documentation from their current school, confirming that they will have completed the Year 9 syllabus.
Sounds as though she's been accelerated a year so assuming she's completed Year 9 there is no reason she can't sit the Year 10 entrance exam.
Idk that you will have preference over this yr10, but Im assuming that JMSS will question why she is applying to enter year 10 at JMSS as she would've already completed year 10 at her current school. She will also be the same age as half of the year 11s. The accelerated bit in that guideline is about people who are quite a bit younger than others in their year level and I would not know if it applies to people who would've already completed the syllabus of the year they are trying to enter.
Still, dw, you've got a bit of time. The year 10s have their exam this week.
Good luck!