Most selective schools do not offer Standard English for a variety of reasons (VERY BAD ALIGNING MAKING IT ALMOST IMPOSSIBLE TO GET A BAND 6 and HIGHER QUALITY OF STUDENTS). I'd assume it'd be the same with Hurlstone given its selective nature, which would explain how you automatically get in.
I go to a selective school with the same "minimum English Advanced system" and you don't have to sign-up for it or anything, since that's the only level of English you can choose, so there's no choice involved. However, you can CHOOSE to do English Extension 1 if you enjoy English, for which there is a "sign-up and they check your English ability" and only then you're in.