^ going to have to agree with above poster that it is (while not impossible) suicide. after seeing what people in my year have gone through with just 13 units, i wouldn't recommend it.
student A: a friend of mine, started with 13. ended up with so much work that he dropped to 11 units. even then he didn't do his extension history trial, didn't do his drama IP trial which was worth a massive amount of his internal. he then dropped extension, but because he only has ten units now drama has to count. so his ranking for drama is shit, as is his english ranking, because he didn't have time to focus on his assessments.
student B: still doing 13 (or may have dropped SAC, can't remember) ended up so stressed that she got really sick. but she still had work to do so she continued doing it, ended up in hospital with pneumonia. she had to get estimates for all her trials, and she's STILL sick - all from the stress of 13 units.
think ahead. it's not just classwork, you'll have to do assessments for EVERY class you do, as well as the HSC at the end. in my opinion your time would be more valuably spent focusing on all of your subjects, i.e. doing really well in a few rather than average in everything because you don't have the time.