I holistically agree with Jazz, but I have to bring up an important point elsewhere: I do know someone who was it the bottom 15 at a selective school for maths, english and junior science in yr 10, came out of no where and scored a 99.65 and 97th percentile UCAT. 99 ATARs come in all shapes and sizes, there is no 1 formula for getting there, or 1 method of doing so. Everyone has their own approach/journey
Yr 11 is the time to truly start focussing. I made the mistake of thinking that fantastic marks in maths and english in years 7-9 was important. And yes, having first place certificates and college medals for high school subjects is nice, I found that irrespective of my ATAR, I would have benefitted more from just understanding and being solid on content from year 7-9, and rather forming better time management and study strategies during this period. I felt as if this would have given me more energy to work harder in year 12, as I found I became consistently over-worked and exhausted with my infatuation of coming first, which had really begun for me from the start of year 8.