my main friendship group in high school of about 9 of us got atars ranging from 90-94 with a 98 and an 85. the guy who got 85 or so actually peaked in year 11 doing two 3/4s with scores over 40 but then dropped off in year 12. the guy who got 98 studied pretty hard the whole way with a 40+ in year 11 and scores all in the high 30s low 40s in year 12. the rest of us from 90-94 did not do much work in year 11 at all other than for our 3/4s where we all got 40ish (+/- 3).
In general, in year 11 we all failed some sacs and switched subjects around. The thing is though, everyone still studied, except that it was focussed on our 3/4s and math and science subjects, and let other subjects fall to the wayside
year 11 is really good for figuring out which subjects you do and dont like, as your grades arent super important. you can definitely not do so well in year 11 and do well in year 12
BUT big changes in scores and study habits are very hard to happen instantaneously. its all good and well to theoretically talk about going from a D student in year 11 to an A+ student in year 12, but the reality is, generally, big changes in grades only come with big changes in study habits. it is incredibly difficult to go from a bludger in year 11, then waking up in the summer holidays and saying "ok time to study 5 hours a day". every year we get tonnes of people on atarntoes saying "i had a 41% average in my 1/2 can i still get 45+ in 3/4???" and the answer is technically yes you can, but realistically, something has to change for your mentality to change from "claw at every mark i can find" to "dropping a mark is unacceptable".
when you look at it from an outsiders perspective, it seems alot more achievable than it really is.
at my school for example (select entry so a bit different to most public schools) you would see HEAPS of students get shitty grades in their 1/2s, in year 11, even fail some subjects, and then get scores in the 40s in their 3/4s. alot of people think "oh they didnt study that much, most of the grades are bad". that is a fallacy, in general there was a shitload of study going on, its just that it was all focussed on 2 or 3(or 1) subject(s)
also, alot of subjects build from 1/2 knowledge, and you really should study for them in year 11. the first one that comes to mind is methods, if you scrape passes in year 11 you will probably get similar/worse scores in year 12.
TL;DR yes, but dont try to do it, start studying now