I believe that this is a matter that calls for consideration of your priorities and your ability to effectively manage your studies in addition to whether going to the concert would be something that you could do without negatively impacting your study strategy and your studies as a whole. There are students at both schools and universities that are generally capable of maintaining some form of entertainment despite their potentially heavy workloads. This is because such students usually have good study strategies and time management, both of which would ensure maximum preparation for their exams, while still allowing them to allocate some time towards entertainment. Based on this, you should consider whether you have been capable of doing this previously and whether you can justify attending this concert, especially since favourable performance in trial exams can significantly improve a student's rank in each of their subjects, which in turn means that you should consider your current position in each of your subjects. If there are no issues, then there does not seem to be a reason not to attend.
On the other hand, there are students who either cannot allocate time towards entertainment (that is, without negatively impacting their performance) and would prefer to spend all available time on studying or are just reluctant to take what they may perceive as risks and would rather use all available time to study. If you feel like this applies to you, then you should probably skip out on the concert. There is also the fact that this is your most important year of school. This suggests that you could go to another concert at a later time after you complete your HSC, which eliminates all risks.
If this were me, I would probably study and skip out on the concert, mainly in order to avoid any "I wish I had studied more instead" type of situation. Even if I fail to perform at a level that would make me satisfied, my conscience would be clear in such a case, because at least I would know that I did my best.
I hope this helps!