Well, if your school traditionally produces a good amount of band 5's with a rank like that, that's your best bet to gauge off your ranks alone. BUT, if your wondering whether you can achieve a high band 5 I would say it's more than possible. I've had many students move from low band 4's all the way to a high band 5 by the time they finished their HSC. Obviously, they worked very hard and did everything they could to improve their marks, if your up for the challenge then it's not hard at all to get that mark.
I suggest you concentrate on 1. moving your ranks higher and 2. finding out why or where you're losing marks and fix those errors. If you constantly aim to get those two things done every time you tackle advanced English your setting yourself up for a good mark.