<em>An overgeneralization is the extension of an application from a set of individuals to every individual that shares a set of characteristics, when in reality, this extension is without merit.</em>
In the case of this sentence, somebody claims that every high school student is a devote philanthropist willing to do their best to make the world a better place, when in reality, not every student shares this trait or is willing to help this cause. Therefore, the speaker's claim is discredited of any value.
