Poverty in family compels many parents to withdraw their children from school and get them employed on petty jobs. This was a topic in one GD. There may be other causes like excessive interests in other pursuits like sports, music etc.
Poverty and the pressure from family due to any reason are the main and most important factor that force about25% of children to leave School in the middle.
Poverty is of course the main reason......However some leave it because they give priority to other work rather than educating themselves.......as in case of business......