I graduated from St Marys 2 yrs ago so here's my 2c from what I can remember
Now to get in, you just hand them in your application form and reports/NAPLAN/other docs etc and they'll assess it and will let you know, so no need for exams or anything. Now me being so bad at English, couldn't get into other selectives (penrith girra etc) so I applied for st marys very late (forgot when) and they said that the chances are pretty small, but eventually got in haha. So as long as you have average to good academic results and good EC's, then you'll defs get in
The school itself is pretty average and is ranked around ~150. They excel at the humanitarian subjects (legal, eng, bus etc) and iirc, they had a couple of state ranks for legal, eng std, general maths etc in 2016
However, the STEM side is a bit average I'd say and only a few gets band 6 for the science subjects (I think I was the only one with b6 phys and chem in my year lol), but the math side is a bit better (there were some really enthusiastic students from my year and 2018)
The teachers are average - good. I was super lucky I had the best ones, esp my math (but she left rip) and chem (gave me extra resources/textbooks to go ahead during the holidays, and even allowed me to skip classes and work on past papers in the library HAHA). So ye the majority of the teachers are really good (although a lot of em are leaving now) and you can also call them by their first name and the student - teacher relationship is super chill too
However, I wasn't a big fan of the pacing of content as I found it pretty slow (might have changed now), especially the science subjects, as we literally had to self learn 1 or 2 modules as part of research tasks and the last module of chem during the last few weeks of yr12. So yeh in that respect, I guess it's like uni, a lot of self learning needed and they do emphasise the 'independence' a lot
aand yeh also no uniform, and being yr11 and 12 only, the cohort is pree large and also very culturally diverse, and most of the student are pree friendly and easy going
also the schools literally a 5 min walk away from the train station (if you take public transport), although the area is a bit dodgy haha. And you also have a small mall (st marys village) with woolies, maccas and all the major stores within a 5 min walk from school
I would say definitely try other selective schools first, and like others said, your experience will really depend on you and if you do get in, make sure to utilise all the available opportunities
Feel free to hmu if you have any other questions or anything!