Mate I'll fill you in, it is confusing though.
There are 3 main comps, GPS which is the premier comp and fields the most talent, CAS who have 3 very talented schools and then 3 not so talented schools who every once in a while field decent teams, and then the ISA, who is weaker than the other 2 comps and apart from 1 or 2 schools every year, really struggle to field ample talent.
The GPS is made up of St Josephs, The Kings School, Scots College, St Ignatius College (Riverview), Shore, Newington College, Sydney Boys, Sydney Grammar and The Armidale School. Only Joeys, Kings, Scots, View, Shore and Newington field 1st XV's and 2nd XV's. Sydney Boys (academic selective school), Grammar (academic selective school) and The Armidale School (rural school) don't field the talent to compete, and hence their 1st XV's take part in the 3rd XV comp. It really is an issue for some of these schools even in this competition. I don't believe Sydney Boys even scored a single point this year in the 3rds comp. However, they haven't necessarily "lost" these schools to different comps, just merely their rugby programs have been left behind.
To further complicate this issue, even Shore have really struggled in recent history, and hadn't won a single 1st XV game in 6 years until this year when they placed 2nd, a huge achievement. This basically meant 5 good rugby programs in the GPS, with one punching bag (Shore), and 3 other programs completely left behind.
To amend this issue, the GPS and CAS combined for a couple of seasons in a sense. This also fixed an issue in the CAS, as Waverley in the CAS had historically been great, as had Knox, and Barker had arguably been the best 1st XV program in Australia, and almost objectively the best 1st XV program in NSW the last 6 years. However, on the other side of the spectrum, Trinity and Cranbrook were often getting smashed, and Aloys was being absolutely bullied, including a scoreline of 95-7 against Barker in 2019, where Barker benched half of their team at halftime AND stopped the game 15 minutes early due to a brawl. It would have been a 150ish scoreline had this not happened. Interestingly enough Aloys scored first this game.
Anyways, this fixed some of the matchup issues for a few years, but after the resurgence of Shore, the GPS have decided to split off again into a home and away style competition only involving the top 6 GPS schools.
We have a real problem in Sydney with these matchup issues. Real Shame.