It is also fair that when someone proposes a certain set of objectives, and then declares a plan to accomplish those, that you can criticize the plan to accomplish those objectives as being less than optimal. IMO, since this is his own money, he has every right to pick the objectives he thinks most important.
I think a marginal $1b spent to end the drug war would do more for his stated goal of fixing underperforming urban schools than a marginal $1b spent on teacher measurement. The irony is he wouldn't even need to spend $1b to make a huge impact on ending the drug war -- just showing through data that the drug war is responsible for many of the problems in schools and urban culture ($10mm to study this?) and then having the courage to promote that result (free or priceless) would accomplish it.