It really depends on your school and what you define as 'hard'-because sometimes even when an exam is equally difficult or harder than HSC trials, whether the school chooses to mark less harshly or more harshly compared to actual HSC standards can have an influence on the student's perception of the trials in comparison to the actual HSC, but generally, unless your school makes easy trials, your performance in your trials can be a good indication of how you will go in the HSC (with improvements of course).