Bill Gates on Monday released his first annual letter from the Bill & Melinda Gates Foundation. The former Microsoft chairman was encouraged by buddy and big donor, Warren Buffett, to start penning his thoughts on philanthropy.

In the letter, Gates was measured in his assessment of the foundation’s progress. In one of its biggest endeavors, high school education (over $2 billion spent so far), he wrote: “Many of the small schools that we invested in did not improve students’ achievement in any significant way.”

One reason: lousy teachers.

Gates, naturally, has looked at lots of data and a new area of interest for him is teacher effectiveness. He started with a data point: “Research shows that there is only half as much variation in student achievement between schools as there is among classrooms in the same school. If you want your child to get the best education possible, it is actually more important to get him assigned to a great teacher than to a great school.”

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