hey don't worry! i actually moved schools at the start of this year, i got in through sitting the hast exam for year 10 entry (yes i still want to move schools again hahah). I was exaggerating a bit about the HAST exam tbh. I'd say it's just the selective test but modified to your current schooling level.
Some tips/insight for the HAST year 9-10 level though:
definitely work on your time management --> some sections of the exams are crazy. the abstract reasoning was 30qns in 30min LOL I ended up performing average in that.
But for the other sections: the writing (20min) , the 2020 qn was relatively easy it was "if you could choose to live in any fictional world, which one would u choose and why" --> you could write in any style you'd like, though I went with an essay and I scored the top band
The maths is not as math-based as you think, it's really just simple mathematics like percentage calculations & stuff + analysing data (no calculators though)
And the reading section is fine. Just make sure you're not tired on the day otherwise your reading & interpreting speed will be affected
Good luck! Although I think most schools might be cancelling the HAST exam this year.