It sounds like you have reached your own saturation level of frustration. Before you spend any more time on, or take on more responsibility for your sister, be sure to realistically assess your own progress and goals. If they start to suffer, it's time to attend to yourself. You are not abandoning her and neither of you should take it that way. I imagine that your younger sister is more in need of a sibling's support rather than 'another' adult 'harrassing' her.
High school tends to be a big adjustment for everyone. If your family situation as also changed, or had high stress incidents in the past year, these too can prevent a 'normal' person from excelling or even coping with day-to-day survival. When mid-term marks are presented in black and white, she'll not be able to avoid recognizing that her current study strategies might be lacking and that there is still time to try a few new ones before year end.
Working with a classmate on homework might be of benefit. In this non-threatening atmosphere, mistakes or mis-interpretations can be corrected without fear of ridicule or failure. If this can't be done in person, there is always the telephone, or even MSN.
A great way to meet more people at school is to join a club or team. One of the hidden benefits of such a committment is that it forces one to be more attuned to your personal time management. With less "free" time, you tend to work smarter with the time you do have. Also, sports get the heart pumping and you think better with that extra oxygen circulating.
If your step-mother hasn't already, it's time she makes appointments to speak with all 8 teachers, with or without your sister. Afterwards, if there are common themes emerging from these authority figures, that's what the two of them - your sister and step-mother, should start addressing. If the high school has a guidance department, even making a plan of action does not have to be done alone. There is always someone around to help.
Remember, always, to celebrate even the tiniest steps in the right direction.