Is there a reason why you want to transfer to baulko? I know a lot of people who did that transfer and regretted it because they didn't like the social life and school vibes there.
The only good reasons for transferring are better STEM subjects, because HGHS is weak in them, and maybe extracurricular sports.
If you really want to apply, though, I think academic comps (e.g. maths olympiad, AMC, ICAS, etc), sports and leadership are good extracurriculars to have. However, they're not crucial because the people I know who got in from Hornsby didn't actually do that many activities; they just had good academic reports