y8 entry is extremely difficult as not a lot of people will be leaving. as someone whos been in gifted and talented and is currently in the selective stream of a partially selective school, gat's competitiveness is in no way near selective schools + they merge the classes in hsc years.
u should be fine i went to the kid laroi's concert the day before my maths exam and still topped so it really comes down to time management
however do lmk if ur not going cuz im looking to get some tickets for that concert :)