Lol definitely not. Umm tbh I was one of those guys who did mediocre during prelims but started picking up their game during the HSC, and even then I didn't consistently study (so not a particularly good representation of the studious people here on bos so don't follow what I did LOL) It was quite the opposite since in year 11/12 I ended up becoming much more social and had a decent amount of ECs. (debating, mock trial, state/regionals swimming, prefect team, etc) IMO it's not too hard to juggle a social life while doing well in the HSC, just depends on how you manage your time I guess.
If I could time travel to year 11 me I'd tell myself to really focus on English (to improve overall skills) and Maths (30% of the HSC exam can include prelim content) as well as work on my essay writing/structure for my Humanities subjects (so basically try in everything lol). I'd also tell myself to try experiment to see the ways that I could study most efficiently (because I didn't do too much during Prelims this was mainly left for the HSC). For me, it seemed that typing up really summarised notes and then abusing my highlighter was the best way I learnt, but for others reading out loud, creating study cards, handwriting notes or just reading the textbook continually may be more effective options.
The school can highly discourage you to not pick up a subject but they can't stop you.