By that I mean that you need to read carefully what the school expects of its applicants. How do you find that out? You can always ask the school, but the best resource is definitely an educational consultant. He will have up to date information on schools' current acceptance practices and much more.
Back to credentials. If you are applying to Grade 10 which is what most people do, then the school will want to see certified transcripts of your previous three years academic work. It will also require recommendations from key teachers such as your English and math teachers. What is the school looking for? Gaps in your learning, your achievements, but most of all, your preparedness for the rigorous academic work any good private school offers.
Standardized admissions testing is one of the assessment tools most private schools use.Taking the SSAT/ISEE in late fall is part of the drill.
Next you need to visit the school - you can even spend a day or an overnight at many schools - and meet with the admissions staff for an interview.
If you have relatives who have attended or are presently attending the school, you will be considered a legacy. A legacy almost always gets preferential treatment simply because the school knows the family.
The last method for getting into a private school is not talked about openly, but it can be effective, namely, buying your way in. A lot depends upon the size of the gift and the need of the school. Crass though it may seem, money does talk.

