What can I do if I have a massive personal problem with a future student? I am a teaching assistant for undergraduate engineering classes. I will be in charge of running lab sessions, helping students, and assigning grades. I have an extreme and insurmountable personal problem with one of the students in one of my lab sections scheduled for next semester and am concerned with my ability to grade them fairly. I will be talking to the Professor of the course about the issue and would like to approach them with different options that would be beneficial for both myself and the student in question. Ultimately the question is: 

What can I do if I have a massive personal problem with a student in one of my lab sections? 

edit: the nature of the problem is that the student violently assaulted my girlfriend, I retroactively see why the nature of the problem is important to the answers for the question. Charges were filed and I stand by the seriousness of the accusation.
the nature of the problem is that the student violently assaulted my girlfriend [...] Charges were filed and I stand by the seriousness of the accusation.


The fact that there is a criminal case involved makes this situation much more serious than just disliking someone. It doesn't matter whether you think you could behave impartially or not, the appearance of a conflict of interest is what is important, and that is most certainly present here. If the student is doing poorly, the student could accuse you of unfair treatment regardless of your behavior, and there would most likely be an investigation. It's better to avoid that possibility entirely.

You need to talk to your professor immediately. Depending on the status of the case, tell your professor that your girlfriend and the student are on opposite sides of a criminal case. This should convey the seriousness of the conflict without disclosing too many personal details. Keep in mind that there are privacy issues with saying too much about your student's personal life, in particular when it comes to criminal charges.

Work with your professor to figure out what the best solution is, whether that means switching sections with another TA, switching the student to another section, or putting in place defined procedures to prevent bias on your part (such as anonymous grading). It may be a good idea for your professor to involve an ethics adviser from the university, since the conflict of interest is so clear. Regardless of the outcome, by officially reporting this potential issue ahead of time, this will reflect positively on you in the event that the student does make any complaints.