Originally (40 years ago when the school began - before that it was still a school, but it was Carlingford High School or something and wasn't agricultural) the school was an agricultural school for those who wanted to do agriculture, ie. it had a farm. It's like technology, sport or arts schools.
For some reason the agricultural schools (Ruse, Hurlstone, Farrar, plus any others I may have forgotten) got to do the selective school tests with the selective schools. So James Ruse isn't a selective school, technically speaking. At the time there were only 6 selective schools (Sydney and North Sydney Boys/Girls, plus 2 more, Fort Street was one I think) and they were all around the city, whereas Ruse was one of the few that wasn't. I don't know how exactly it became the best school academically, but that is why it is an agricultural school.
The lowest UAI I heard of (at Ruse) was around 60, but that's just one I heard about and I can't really confirm it with much certainty.
I think the person who got the lowest UAI from recent history at James Ruse is Jabba from channel V (if any of you get pay TV). I don't know what he got, but it was really low and he didn't go to uni, but ended up getting a job on channel V and is probably the second most famous ex-Ruse student (the most being Catrina Warren from Harry's Practise).