Wow, I don't think i've ever seen my school mentioned on here before. St Paul's is a pretty good school if you ask me. I'm assuming you're going to do the HSC (the IB is also offered at St Paul's), so what I say here is based on my experiences in the HSC, but I wouldn't think there would be much difference in the IB.
The current Year 10 (which will become the Year 12 2014 cohort) seem like a pretty good grade to me. I have a few friends in that grade. I won't lie, there are some bogans/drop kicks in that grade, but less than most other schools in the area (other than perhaps Penrith High). From what I hear there are around 70 students in the HSC cohort for 2014, with almost all subjects running.
As for teachers, i'm not going to disclose the names of any of the teachers (one thing about St Pauls is that they are very concerned about their image, students have gotten in big trouble for writing things about teachers on Facebook and Wikipedia in the past), but I can give you a heads up on what the departments are like:
Humanities (History, Business, Economics, Geography etc) - Mostly good, with a couple of exceptions.
Sciences - It's either great or terrible, no in between. There's some brilliant, dedicated teachers in Science, and some awful ones.
English - A pretty good department, again with one or two exceptions, but there aren't any really bad English teachers.
Maths - Awesome. Can't go wrong with Maths teachers at St Paul's.
PDHPE - Pretty bad, with one or two exceptions.
Languages - Spanish, Chinese and Latin are terrible from what I hear, French is varied (there's one amazing teacher, and two awful teachers).
Visual Arts, Performing Arts and Technology I can't comment on.
If you want to know about a specific teacher (or teachers), pm me.
The student culture at St Paul's is pretty good, it's a small school (650 students) so you would know everyone in your grade. One of the great things about St Paul's is that it's socially acceptable for seniors to be friends with and talk to younger students (with the exception of year 7 and 8), as long as you have friends in your own grade as well. There isn't much bullying going on, and there isn't much of a social divide, most students, regardless of what 'group' they belong to, get along fine (again, there are a few exceptions).
Hope I helped, PM me if you want to know more.