No argument, we all know that. However, the education system in America is not the cause of leftward thinking, but the result. How do we change that dynamic? Certainly not by voting for politicians.
It was a supposedly conservative Republican which gave us the Dept. of Education.
I believe it was Jimmy Carter who did that, as a campaign promise to the teachers union. It effectively gave them a cabinet post.
He signed the Public Education Organization Act creating the DOE in October 79, and it went into operation in May 80.
I'm not sure it is the result as much as it is the vehicle for propagation. One can't prevent dangerous ideas, but you can control the contagious spread of ideas. Some bad ideas will simply die out in the market place of ideas, unless they are foisted upon a population with force. When the government taxes a population in order to force those bad ideas on that population you have a tyranny called government schools.