It is hard, but not impossible. Here are a few things teachers can do to not be prejudice.
Stick to the facts. If there is a problem, record the problem, events, or situation write down just the facts. Keep emotions out of it. For example, if you don't feel that a child behavior is unacceptable, then write down exactly what happened, what is said and the outcome. Records should include names, dates, times and witnesses. If a rule was broken, state the rule.
Have someone else complete the evaluation. There are other staff members that can collect the data and make a recommendation or decision.
Talk to someone else. Find a trusted friend or colleague that you can bounce your experience off of. Have her look at the evaluation and ask how she would interpret the data. Ask this person to give you their honest opinion. This level-headed person can tell you if and when you are being bias.
Reflect on your actions. Have you be accused of being bias in the past? If so, then take a close look at yourself. This might be difficult, but not a lost cause. If after close examination you see that you are bias, then get help. Most insurance programs have employee service programs which offer counseling. Don't be afraid to use it. You can work on correcting the problem and become a better teacher and person in the future.
If this doesn't answer you questions, please give us a little more information.
I'm sure it's really hard for teachers to avoid letting their personal feelings about a kid influence the grades they give that student. And I'm sure some teachers are better at it than others. But I think that's why schools, school districts, and states have a standards based approach to giving grades. At my kids' school there's a very detailed description of each item on their report card . If they can do that thing well, they get the highest grade, if they can't do it as well they get a lower grade.
If you think your child is being graded unfairly, I wouldn't hesitate to discuss it with the teacher and/or principal. The teacher should be able to provide you with a clear explanation for why your child received the grade he did and what he can do to improve that grade.