If you are speaking about dropping out, as the other posters have suggested, I would say that it is a combination of many things. For example, in the rural area where I teach, many students know that they have a job to walk into. If the family is having problems, and the student can help by dropping out, the student will drop out in order to help the family business.
As pointed out by wannam, other students simply see no importance in education. Instead, they value other things (like money, or drugs, etc.). They do not value education because they believe that it will offer them nothing at the moment.