I go to Girra and there's a really broad range of people. In my grade it ranges from the white guys that are naturally intelligent but do no study and spend their time drinking and smoking weed to the people that aren't that smart but spend all their time studying/getting tutored and end up getting fairly good results because they rote learn everything. Then there's the well rounded people in between that are smart and study - these people do really well.
In general selective schools do fairly well - If you look at the school rankings this year most of the top schools were selective. If you've got the opportunity to attend one, I'd recommend taking it. You're in a competitive environment, which pushes you to do better, but (at my school anyway) everyone's fairly friendly and supportive as well.