The cost of extending the period of compulsory education is just too high. In many countries the number of students in the last two years of formal schooling would at least double, requiring a huge investment in teachers, books, new school buildings, computers, etc. And this is just the direct cost - there are also potentially enormous indirect losses to the state in terms of the taxes and pension contributions which it currently receives from young workers but would forego if the school-leaving age was raised.