Wikipedia, in an entry on “raising of school leaving age (often shortened to ROSLA)” reports that 15 states and the District of Columbia have already raised their dropout age to 18. And it has the kicker that helps explain what may be a factor motivating Mr. Obama on this one: “The National Education Association, the main teachers’ union in the United States, advocates requiring students to earn a high school diploma or stay in school until age 21.”
I read that sentence in Wikipedia and thought to myself, “oh, that explains it.” A Bloomberg article has details, including the news that the teachers themselves estimate that the compulsory education until 21 plan would cost an additional $1 billion a year, much of which would naturally be spent on unionized teachers paying dues to unions that reliably support Democratic candidates.