OK, you've almost got it - it has NOTHING to do with your SCHOOL assessment mark, only your moderated assessment mark &, as you are ranked 1st you will get the highest exam mark (aligned) for this mark. Now, there isn't a lot of IPT raw info to go off but I know that a 64 raw last year aligned to a 79. That's a high band 4 right there. Your final band will depend, as I said above, on the highest exam mark as you ranked 1st. So, if the top raw mark in your class is high, it is feasible that you could walk away with a high B5. You'll have to wait for your results. However, if you believe YOU have received the highest exam mark in your cohort, you will walk away with a B4.
I hope that makes sense...