I don't study education, but I have seen these kinds of statistics before, and it always depresses me....
"Research indicates that persons entering the teaching field had lower SAT scores than their non-teaching, college-graduating peer and those leaving the profession had higher scores than those remaining as teachers. In addition, the scores of those majoring in education were lower than those of graduates who majored in another field. Those who came to teaching without prior preparation, such as student teaching, had higher scores." Source: Social Change in America, The Historical Handbook 2004. Edited by Patricia Becker.
K-12 teaching is as difficult as it is important, so it's a shame that we can't keep the smartest teachers. I've read before that this is an argument for raising their salaries... to keep the best people (who are presumably also the smartest, though, perhaps not).