Totally apolitical, there is not just one teacher who influenced me at secondary level, but if I have to narrow it down there were four of them - one moreso than the others.
We had a teacher who took us for English and knew him as 'Mister Boar' (I think that's the right spelling). What makes me remember him in particular is that he read from the BEOWULF saga, several pages at a time. I don't know what my classmates made of it - we were in a mixed state school - and I'll warrant several had him down as 'Bore'. I was more inspired and carried it with me. I have two copies of the book, one by Seamus Heaney with the 'Fight at Finnsburh', the other is the actor Julian Glover's version illustrated by Sheila Mackie and forward by Magnus Magnusson. I write historical fiction these days since 2008, and have written Hub pages on early English Mediaeval history from the post-Roman Migration Period.
The other three were a woodwork teacher, a history teacher and a geography+maths teacher, mostly because of their personalities. The woodwork teacher died some years ago, the geography+maths teacher lives in the Yorkshire Dales with his wife and I look in on them when I get up there. We're friends, although I was never much at maths. He couldn't understand why I was good at geography and not maths, but that's passed under the bridge along with the torrent of time and tide. I lost touch with the history teacher, who later became a head master in the same area, although the influence is still there.
By the way each of them was ex-Services, Army and RAF, and didn't need to throw their weight about like some teachers (or managers in subsequent times) to get things done..