It's unfortunate because it's a flawed process, because the achievements that they value heavily favours students from private schools: where a breath of co-curriculars are offered, where you are sure to have debating teams and coaches that inspire you to be good as well as a performing arts and music program. Obviously, there are opportunities for students to pursue these activities outside of school and become good at it, but this kind of proactiveness at a young age often relies on parents' encouragement (and at times how much money they have).
I guess in any situation, you have to do the best with what you've got (are are doing). As I said before, it's not about what you do, but your commitment to them. The school can tell the difference (since all students are the school have to do co-curriculars and the commitment level obviously varies as a result) between doing something for the sake of doing co-curriculars and being committed at it. The amount of opportunities that people have to pursue co-curriculars (and to become good at it) varies, but you have to make your commitment and the way they have shaped your personal development clear to the school, as well as how the programmes the school has will provide you more opportunities to further your interest/capability in those fields.