The first thing you should do is narrow your topic. Paradoxically, it is easier to write a long speech on a narrow topic than on a broad topic. If you try to talk about racism in general, after saying that stereotyping people by race is both inaccurate and morally wrong, you don't have much else to say. To say more, you might want to focus on one specific type of racism or prejudice against one specific group.
One way to write about this would be from personal experience. Think about examples of racism you see in your school. If you look carefully around your classroom or the cafeteria or the playground, you might see that children of similar ethnic backgrounds tend to hang out together. Then you might think about whether the neighborhoods in the area in which you live are divided by race, with certain neighborhoods being predominately black and others Hispanic and others Asian or white. Next you might even look at social groupings in your school. In your speech you could summarize all of these observations and talk about how these might result from subtle forms of racism. Then you could conclude your paper by some sort of call to action, for example, looking into creating "multi-ethnic" tables in the cafeteria with the goal of every student sitting next to a student from a very different background at least once a week.