Stereotyping is something that goes on daily both inside and outside of the classroom everywhere. As secondary teachers I think we may even do it more. We become fearful of certain students because of what we've heard about them, how they dress, and even though we won't admit it..for their ethnicity. I found myself my first couple of days stereotyping a lot. Through observations there were students that I felt were only going to cause problems, and it made me less apt to check on them through guided practice. We can also stereotype students whom we perceive to come from great backgrounds. Sometimes these students are the ones that are dealing with many struggles.
One tip my teacher gave me is to never show your roster to other teachers. When you share your roster, teachers will give you horror stories about certain students. These stories put a perception in your head about these students when in actuality they could be completely different for you or may have just had a bad year before.
In student teaching, I discovered to pay attention to the students that are working hard. Of course encourage and engage ALL students constantly and consistently, but no matter what kind of student they are if they are working hard for you, they deserve your time and attention. A positive outlook towards all students is also a great tool to prevent stereotyping. I try to think of at least one positive attribute on each student before working with them. It has helped me a lot!