if ur getting 80% in school tests, that isn't low bad 5 level, that's high band 5 or even a low band 6.
This depends on how hard your school's papers are. They might not always reflect HSC level difficulty. However, since eggs only does high-scaling subjects, a mid band 5 raw could very well be scaling to a low to mid band 6 depending on the subject
That being said, it's much more reliable to look at your rank in your subjects rather than your marks - how many people usually get a band 6 in your school for each subject?
But to directly answer your question, yes, you definitely can get a mid band 5 assessment mark and a low band 6 HSC mark (it's just the average between the 2 marks anyway), but it'd be hard. Say if you got an 85 assessment mark, then you'd need a 95 exam mark to get a 90. That kind of disparity is rare, and you'd be better off trying to improve your rank in the upcoming trials. I do know someone who got an 88 assessment mark but a 95 exam mark in bio, and ended up with a 92 HSC mark