As I mentioned previously, this sentence is about what was to be expected, compared with previous reckless homicide cases in Indiana and taking the sentencing guidelines into account. I've written a post detailing other cases earlier in this thread, so I won't repeat those, but it's clear people who behave far more recklessly (DUI, excessive speeding) than this woman also receive short sentences. The one aggravating factor that really gets you slammed with the max penalty is having priors/being an habitual offender. Looking at the facts of the case and looking at other reckless homicide cases, it's readily apparent that at no point would she ever have gotten the max and that the sentence given falls well within the expected sentencing range.
In this case the incident happened before sunrise on a dark country road at an unilluminated bus stop (which parents had complained about multiple times in the past) , where kids were forced to cross a 55 mph road in the dark. The defendant was not driving under the influence of either alcohol or drugs, wasn't speeding and wasn't dawdling on her phone. As noted, the defendant said she thought it was a semi or some sort of farm truck, which is, given the time of the year and the area, not such a strange thing to believe. I'll specifically point out that at no point during the trial did the prosecution try to make the argument that she was lying and actually intentionally passed the school bus and that the behavior that she got convicted for was not 'knowingly and intentionally passing a school bus', but instead 'not reducing your speed whilst passing a large vehicle with flashing lights'. As far as reckless behavior goes (compared to other reckless homicide cases), that's at the very bottom end of the bell curve, and that fact alone was enough to ensure a sentence at the low end of the scale.