It is not very uncommon to see withdrawn and shy kids in schools. These kids are aloof and somewhat indifferent to other children. Most likely, these kids seem unhappy because they are being bullied at school.
Being bullied can be a total nightmare for any child. Although bullying can also be experienced by adults, this kind of social act frequently happens among kids or adolescents.
Bullying is different from an innocent teasing of a classmate, friend, or a sibling. Mere teasing is not harmful when done in a humorous or in an unintentional way. However, when teasing becomes extreme to the point that it is hurtful and turns into a habit on the part of the teaser, this act is then considered bullying.