If something is prejudicial, it's unfairly biased or damaging. If you've already decided that you don't like your new biology teacher before you've even met her, you're treating her in a prejudicial way.
While prejudicial can be used as a synonym for prejudiced or biased, it's more commonly used to simply mean "damaging." A child's temper tantrum is prejudicial to the teacher's control of a preschool class. And a negative opinion piece in the newspaper about a local car dealer might be prejudicial to its business — in other words, it might hurt car sales. The Medieval Latin root is prejudicialis, "injurious."