A few gems from a secondary maths teacher:
"If you'd pushed that chair in, you wouldn't have to climb over it."
"Go into your attic, you will see right-angled triangles everywhere."
"Whenever my maths master drew a circle on the board, he used his tie as a compass." With the class egging him on, he tried to do this himself, but it was less good than his freehand one!
When writing trigonometry questions, he would illustrate them, perhaps with a triangular pub sign labelled "the happy teacher".
When explaining Newton's Third Law (for every force, there is an equal and opposite reaction): "You know, when they caned you, this is why they'd always say 'this hurts me more than it hurts you'."
"It's the last lesson of the day, and it's raining. While you're going home and I'm stuck here marking, I shall watch you all getting wet, and have a good laugh."
While giving an assembly: "I'm going to talk about a three-letter word that has changed its meaning in recent years. Sad. Yes, you heard me right, 'sad'. My family say to me every day: 'dad, you're sad.'"
To show us that last-minute revision didn't work, another teacher got us to watch five minutes of Blackadder, before giving us a test with questions such as "why didn't Blackadder consider a career in the church?".
Also a Latin teacher gave a whole lesson on one single word. He gave us some paper which said "Have you seen this 'QUAM'", which explained that this word had four different meanings. It concluded with "look out for this blighter, because if you do not, he is sure to cause you bovver and attack your exam mark".