An outcast often starts out in life neglected and severely abused by parent(s) who, even sometimes secretly and unspokenly, reject the outcast as a baby or small child. The trauma of rejection and abuse distracts the little outcast from developing normal social skills while growing up.
Not being accepted at home, the outcast does not learn how to accept their own self. To try to compensate, the outcast tries too desperately to find acceptance out in the world. Naturally, kids at school sense the excessive despair, and sometimes use it to tease and frustrate the outcast. ( "Why should I like you if you don't seem to even like yourself?" ) Some kids take advantage of that vulnerability in the outcast.
The excessively desperate need of the outcast for outside acceptance (even if this becomes a "secret") continues to be met with resistance and further rejection at school. Over the years, this process can have a spiraling effect.
If an outcast also happens to be highly intelligent or extremely talented in some way, this can really complicate the situation if the outcast encounters jealousy in some of the other kids.
Many outcasts in school had started out as severely abused and neglected children at home. I know first hand because I was an outcast.