Kids normally do a great job at accepting other people right off the bat, before they even really get to know each other. This innocence is tainted with experience as they get older, but it's mainly experiences taught to them by others (ie adults).
I'm learning all about how to teach kids with disabilities in my class "Teaching Students with Special Needs." One of the biggest difficulties is dealing with how other kids perceive and treat them because of their disabilities (anything from dyslexia to Down syndrome to Cerebral Palsy). As teachers, we can model appropriate behavior and have a sizable impact. The trick?
Treat a kid like a kid, no matter how different they may be.