The fact that you attend a school ranked 50th, and given your rank relative to your cohort, it would be reasonable to suggest that your performance in English Standard may already be consistent with a band 5. While band 6's in English Standard are significantly less common compared to English Advanced, numerous band 5's are usually achieved in English Standard. Your position therefore seems promising.
In order to maximise your chances of attaining a band 5, you should prepare for your English Standard trial exam to the best of your ability. This will hopefully allow you to either maintain or improve your rank thanks to favourable performance. The good thing about a trial exam is that it allows you to gain an idea of what the HSC exam could be like. This is why it is important that, following your trial exams, you ensure that you do your utmost to address any areas of improvement (including focusing on your analytical ability in terms of questions targeting unseen texts, improving your essays as well as your writing ability for Module C) that are identified from your performance in your English Standard trial exam in preparation for your HSC exam.
The more you prepare, the better you are likely to perform in your HSC exam, and the higher your chances of achieving an HSC mark in the band 5 range.