Teachers have tough, low-paying jobs. If you think about it, they stand up there trying to balance teaching the curriculum, engaging students and providing pupils practical (useful) knowledge. I am not attempting to cast any aspersions on the motivations of teachers. They are all wonderful people (at least most of them, there is always a few bad eggs or apples).
However, the bias of many teachers seeps through their teaching, particularly their political beliefs. In about a week in high school classes, you can tell the Trump supporters from the Bernie supporters. It usually breakdowns with the older grouchy teachers on one side and the young naïve teachers on the other side (I think you can guess which side each falls in). Then, you can tell the ones probably libertarian or even apolitical.
There is also a correlation which goes largely unnoticed with subject taught and political affiliation. It leads to the typical chicken and egg argument, does the subject influence a teacher's political philosophy or are people with certain ideologies attracted to certain subjects. The more legitimate argument seems to be the latter. This can be induced from the non-professional attributes of teachers, meaning people of certain demographics from certain places seem to go to into teaching certain topics.
The correlation previously spoken about is seen when a person looks at certain classes. For example, my experiences with English and Math teachers seem to lean conservative, while their History and Science counterparts are more progressive. This seems easily explainable by the idea that mathematics and English are classes where most of the information taught is set and established. Even if math can be theoretical most of the subject matter being taught is to the tune of "2 +2 = 4". I realize that’s a simplified version of most high school curriculum, but you get the point that there will never be a day (at least in the near future) when that problem equals anything, but four. Also, while there might be many new well-written novels coming out every week, most curricula for years have stuck with teaching students the so-called "literary classics". These "classics" can probably be remembered not for their engaging plot, but the drawn out plot. The idea that the subjects being taught shouldn’t be overly radical or consistently changing seems to fit more with conservative agenda.
On another hand, History and Science attract more progressives because it’s in the very nature of these subjects to progress forwards. Without advancements in science students would still be studying the art of alchemy and learning Pluto was a planet. The same conservative principles mentioned beforehand is still trying to say evolution does not exist. The same idea extends to history where conservative ideas come in contradiction with history leading to the minimizing slavery. To conclude, social sciences and science attract people with a political philosophy more willing to accommodate new ideas.
Also, Spanish and other foreign language teachers are more likely to be progressive, but for a different possibly more personal reason. Many foreign language teachers are of the one or many ethnicities that speak the language they teach. If the teacher is a native English speaker they are sensitive to the plight of the immigrants that speak the language. I assume as a reader your mind as already started thinking of Trump when comes to non-English language teachers being anti-Republican. However, this is not an issue that has just arisen, it’s just alienating the Hispanic and overall foreign population more and more.
English and Math aren’t usually subjects that intersect with public policy. However, History, Spanish, and Science are subjects. Therefore, when Republicans complain about biases in everything from textbooks to the media they have to realize they are fighting a futile battle unless there is a massive revamping of the conservative agenda. A textbook without what would be considered a liberal bias would be providing students with false information and in addition and would not matter because the teachers teaching these courses already have a set of beliefs veering to the progressive side. The media may have a left-wing bias because they have been taught what they are talking about from left-wing teachers who mostly subconsciously realized that one side of the political spectrum is suitable for the success of their area of study.