I personally teach it as 'the direction it moves as the handle passes over your head.'
simple...fowrward over you head - forward trim,
backwards over your head - backwards trim..
Dont like the idea of the students looking up to see which way it turns - i have a hard enough time convincing them to keep looking out of the window!!
The worse thing about the 140 trimmer.....you have to 'search' for it unless you are very careful about remembering where you put it last!!