What are the causes that lead to "leakage of students from schools?"
"Especially in high school.... 0_0 "
What are the solutions to this problem?
I would argue that overcrowding in the classroom is another primary cause, of many problems, not just dropping out. When teachers are in charge of 30+ students at a time (and perhaps three or more classes of this size), it is very difficult to build and maintain the kind of relationships that foster a healthy view of academics, educational progression, and even simply the idea that someone at school cares to see a student succeed.
I think a lot of it has to do with a lack of appreciation for education. The generation of children who are in high school right now, especially in America, are so removed from times or cultures where people will risk their lives to get an education. They see it as something they are made to do, rather than a priviledge that will open doors to them in the future. They have never had the experience of being deprived of an education, therefore, they don't understand implications of not getting one. Furthermore, I think parents fail to get involved in their children's education. This adds to the lack of appreciation because they don't have their parents emphasizing the importance of education. If students have problems with authority, all they see is an adult that they may or may not trust telling them to do something they would rather not do.
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.
Many people enroll in college because they think it's the thing to do as soon as they get out of high school. Often, however, they are unmotivated and unfocused, and so they fail to take advantage of the opportunities college can offer. They are also often distracted by other things in their lives, such as socializing. Some of the very best students I have had have been people who have taken a break from school after graduating from high school. They have worked in the "real world" and then come to college. These students often work very hard and appreciate the value of a college education.
Where I teach, I think that it's a combination of culture and their perceptions of economic opportunities. There are lots of families that don't have anyone who has ever gone to college and so they don't think it's all that important. The kids feel that they will/ought to go into the same sorts of work as their parents. Therefore, they dn't really think that education is going to do them any good.