Less, actually. According to the New York Times, she only served 8 months although she was indeed charged with 12 months at the sentencing (some accounts also render the number as six months served). I have not found any documentation as to how she got early release, but general wisdom seems to be that each day of good behavior acts as two days of sentence for most crimes, so she would only have had to have been well-behaved for four of the eight months if that were the case.
Whether or not she actually planned the murder did not really enter into sentencing since she pleaded down to involuntary manslaughter, which has no minimum sentencing.
SECTION 16-3-60. Involuntary manslaughter; "criminal negligence" defined.
With regard to the crime of involuntary manslaughter, criminal negligence is defined as the reckless disregard of the safety of others. A person charged with the crime of involuntary manslaughter may be convicted only upon a showing of criminal negligence as defined in this section. A person convicted of involuntary manslaughter must be imprisoned not more than five years.
With the court records sealed, it's hard to say why they might have gone with "involuntary manslaughter". Manslaughter (a more likely crime for beating someone to death with a candlestick) currently carries a two year minimum sentence (I don't know if it was different then), so the sentence would have been longer if she had been convicted of that. It's possible, also, that juvenile offenses might be treated differently. Nevertheless, the truth may never be known for anyone not directly involved with the case or with access to those sealed files.