Sounds like you are all set to teach the easy students who would learn with or without a teacher, but not the students for whom a good teacher will actually matter. Stop complaining about how awful she is. You get paid to get results.
1. Assess her current situation, including strengths, weaknesses, and areas where improvement is likely. Be careful not to substitute your judgment for hers, your boss’s, or her parents’.
2. Figure out what a reasonable goal is.
3. Figure out what steps would move toward that goal.
4. Implement those steps and a bit more.