Most of the atar generators are already in the school. Very rarely, not to say never, does a student who transfers rise to the ranks straight away within the school. It's more of a push to become a school that has a holistic approach to character development. Someone who excels not only in academics but in their chosen extracurricular and most importantly shows a desire to challenge themselves (they don't have to produce the most exceptional results), will most likely stand out to these schools, as it shows that you are willing to take on opportunities. This is not to say that academics aren't crucial, but they don't expect "you" to be the student who drastically increases the school's rank. If you happen to rise to the top, great job, but don't consider this a barrier to your selection.