In theory, that would be a good thing to do, as the optimal outcome (i.e. if everything goes as planned) from an internal perspective would be that the relative differences (gaps) between students' Assessment Marks would be reduced, based on the assumption that the cohort would perform at a superior standard in the HSC exams as a whole because the students of that cohort are all helping each other with their studies. In such a case, a lower internal rank would not constitute as much of a problem as it normally would. Coupled with the notion that we are focusing on a school ranked within the top 10-20 (meaning that it would already have an academically capable cohort), this would further mitigate the unfavourable effect of a lower internal rank.
This brings us to achieving this, which is where you may face obstacles. For example, the ideal condition would be that the entire cohort engages in such collaboration, as opposed to some students, in which case the effect would not be as significant. Whether this can be guaranteed may be uncertain, and this may be even more relevant given the fact that we are focusing on a high-ranked school, where a number of students can have a competitive mindset and may therefore consider this as a potentially negative influence on their performance, making them reluctant to assist their peers.
In addition to resource sharing and helping each other with questions (both of which can definitely contribute to superior cohort performance), perhaps you could consider collective exam preparation of some sort.
I hope this helps!