The proposal does have some obvious political advantages, like 100,000 new members for teachers' unions. But beyond that, it raises more problems than it offers solutions. It'll certainly burden school districts and state governments, and the taxpayers who support them, with more health insurance and pension plans to pay for. With no guarantee that more teachers will equal better teaching.
To quote Jay Greene of the University of Arkansas' department of education reform:
"Unless the next teacher-hiring binge produces something that the last several couldn't, there is no reason to expect it to contribute to student outcomes." Why not? As the professor explains, "Most people expect that more individualized attention from teachers should help students learn. The problem is that expanding the number of hires means dipping deeper into the potential teacher labor pool. That means additional teachers are likely to be weaker than current ones." And their students may prove even more poorly educated.
The idea that there is some royal road to improving education, one that requires only certifying more and more teachers every year (not necessarily educating them) has proven one of the more attractive fallacies of recent educational policy. It's especially attractive to politicians looking for simple solutions to offer a gullible public. Politicians like our president.
But as the late great H.L. Mencken once pointed out, "there is always a well-known solution to every human problem -- neat, plausible and wrong."