Did anyone out there get a low sac score (50%-60%) and still managed to get a 99 atar??
The short answer to this is
yes - quite a few people have done this in the past. However, I don't think it is nearly as important to understand whether it is possible as the importance of understanding what you need to do to change your trajectory. The probability is not high because there are a couple of underlying factors that need to be addressed for you to turn things around
1. Understanding why you got the 55% Was it because of careless errors, your attitude to studying, you not spending the appropriate amount of time preparing yourself for the SAC? Inadequate resources or inadequate use of resources? Your school just having ridiculously hard SACs? There are so many drivers behind why you get a 55% SAC score and you need to invest the time to understand why you got this score.
A 55% by itself is not a cause for huge alarm, but repeated 55% SAC scores will probably inevitably mean that you won't even get close to a 99 ATAR.
2. What are you going to do now Once you've realised what is going wrong, you need to make the investment to turn things around. If you're aiming for medicine (which is quite difficult to get into), you need to make a commitment to make better decisions. Figure out what is going wrong and take immediate action to remedy this. Your SAC score does not matter nearly as much as what you do from now on.
E.g. if you're spending 2 hours a day on social media, give your phone to your parents and reinvest that time into studying for your SAC. Get appropriate support to identify better ways to study and perform
Those students who are able to go from ~50% in a SAC and being placed at near the bottom of their year to getting offers in undergrad medicine (I've personally taught a few) are those who take decisive actions immediately to turn around their trajectory and perform significantly better on every SAC afterwards. It is definitely possible but will require a lot of commitment, investment and sacrifice in order to do so. The road is not easy, but it is certainly possible.
3. Invest in best practices I'd advise to learn from best practice here (or those who embody it). In your case, that's working with friends/other students who are on the track to medicine and learning from their behaviours, adopting their mentality and studying with them.
For me personally, in Year 12 during the holidays I would follow the exact same study schedule as my friend who ended up getting 99.95 - I benefited heaps and understood the commitment required to get there (8+ hours of intensely concentrated study every day).
In university, one of my close friends had degrees from Yale and Harvard so I had frequent conversations with her about the application process and understood more about what it takes to get admitted.
Hope this is helpful