American state school teachers may, for all I know, be as consistently left-wing as ours. One needs no conspiracy theory to account for it. People who choose careers financed by force either have no scruples about it when they sign up, or soon learn all the excuses necessary to live with a clear conscience. Such careers are simply not attractive to those of an independent spirit - though many such are to be found teaching in private schools.
It's heartening however that - because so much of it is independently-financed - American higher education (though hardly pure) still has enough diversity to allow the linked exchange between an economics professor and an 11th grader.
Can anyone imagine such correspondence with a pupil in a British state school?