Oh for God's sake. Khan Academy is great complementary source. It doesn't replace schools and teachers. If you're a student and you devote extra time to understanding material you learned in class by watching Khan's videos, you're already on the right track, and you're going to do well because clearly you care. If you gain some incorrect understanding of a concept because the video either omitted some fact, or got it wrong, you'll get it wrong on the test, and you'll learn why. If there's a discrepancy between one of Khan's videos and what your teacher taught you, that's a great opportunity to ask your teacher to explain it.

I don't see the downside to such a great resource.

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Also, there are many many terrible teachers out there. I had terrible teachers at the elementary and high school level. Teachers who were either incompetent or did not care. Khan Academy is not the problem.