Teachers are not all left-wing (to my knowledge) but they are all extremely concerned about funding, as am I, and I am most certainly no Corbynista. I began teaching in 1972 and things are tighter than I remember since the late 1990s (I keep in touch because I volunteer in my local Academy) e.g. 4 T.A.s (all efficient and effective) were forced to apply for 1 job in July. This school has 24% autistic pupils and is supposed to have funding accordingly. Unsurprisingly things are not going too well and teachers are frantically trying to fill the gaps.
That's one side of it. By nature, teachers care about the well-being of their pupils - which tends to put them on the left-ish side. Another consideration is that the profession has changed. Older people find the pace of change and the pressures far too much to live with (a friend had a minor stroke the day before term began in Sept. - the result of stress) and the teachers now tend to be younger and so more inexperienced and radical. There used to be much more of a balance in a school.
Add to this the turn-over rate, where youngsters only stay about 5 years and are then replaced by other 'wet-behind-the ears' (for the most part) cheaper, just-starting, teachers.
Result - a balance in favour of enthusiastic, fairly radical, only-just-ex-students. (Who, by the way don't seem to understand basic English rules - I correct my Grandson's grammar and he says that his, very nice and committed, young teacher doesn't do that!).
I am not sure how this answers the question, except to explain some of the perceived bias. In my experience, the young teachers realise very quickly the realities of life and I have never, ever come across a school where teachers were all Labour supporters. It would be anathema to most to attempt to 'brainwash' - but broad thinking is always to be encouraged - you can't teach one point of view.
Does this help, hope so, I am as disturbed by Isabel's statement as you. That should not be so....perhaps some were in shock? Still inexcusable if true and not misinterpreted.