I find this data hard to believe. I think you will find that ninety percent of those self described "conservatives" vote democrat, and a good chunk of those "liberals" vote for obscure groupuscules like the greens, because they think that the democrats are conservative.The GSS queries respondents on who they voted for in previous presidential elections. It is conceivable that at the state and local levels, where NEA campaign funding and electoral urgings are more acute, this occurs to some extent (though 90% is surely hyperbolic). But when it comes to who is occupying the White House, elementary school teachers vote close to how their political self-descriptions indicate they would.
Averaging voting behavior of the Presidential elections from 1992 to 2004 by elementary school teachers yields 50.8% supporting the Democrat, 43.8% the Republican, and 5.5% a third party candidate. Among the general electorate, 47.2% backed the Democrat, 44.2% the Republican, and 8.6% someone from a third party.
So it appears that elementary school teachers vote slightly more to the left than would be predicted by their stated personal sentiments, but not by much. It isn't difficult to imagine how teacher union 'propaganda' alone could convince a few waffly or generally apathetic teachers to vote for the Democrat because he's touted as being better for educators in this, that, and the other way, when they would've backed the Republican if paying attention.
GSS variables used: ISCO88(2331), PRES92, PRES96, PRES00, PRES04