Let's also not forget that the highest dropout rates are in the worst schools. Even the kids who want an education often graduate from these schools barely able to read. Where does Obama get the idea that the reluctant students, compelled to remain, will reap a rich harvest of learning?
It might be argued that even if there is no benefit from keeping these students around till they turn 18, there can't be any harm. But think again.
The presence of disruptive, unmotivated kids in a class is a drain on teachers, a distraction to other students and a daily obstacle to learning. One of the best things you can do for students who want to do the right thing is to remove those who would rather goof off or make trouble.
It's not clear that laws like this will even work. A 2010 Johns Hopkins University study found that when six states raised the mandatory attendance age, three saw no increase in graduation rates -- and one saw a decline. Coauthor Robert Balfanz praises the 18-year-old mandate, but told The New York Times that "it's not the magical thing that in itself will keep kids in school."
If you want to keep unwilling students in school, you can spend money on truancy enforcement, which means taking money away from the willing students. It would be more rational to use the funds on education improvements so more kids will choose to stay.
A private company -- or a private school -- whose customers are fleeing has to come up with ways to keep them around. In Obama's public sector, there is a quicker solution: Lock the exits.