Spelling tests didn't help me at all. I guess they help some kids, but for me they just made me paranoid about my writing to the point where I just didn't want to do it. My spelling only began to improve in the latter half of secondary school and in 6th form, where a lot of work was done on the computer with the help of my friend the spellchecker. Seeing my misspellings underlined and being able to correct them instantly and see the difference helped a lot, and now most of my misspellings are actually typos.
But that's besides the point. The point was, that there is more to American English than changing S to Z and eliminating the letter U from a bunch of words. The spellchecker helps with that, of course. I just switch it to 'English US' and suddenly those familiar red underlines appear everywhere. The problem I have is when words are different. This doesn't matter quite so much if it's something I write as part of a description of a scene, for example, but if it's in the dialogue it can be really jarring. Even to me, a British English speaker, if Edgar Frog talks about (not that he would) the colour of his trousers, the spelling of colour isn't audible, the word trousers is, and he wouldn't say that.
I've picked out that particular example because over however many years of writing fanfic, I have agonised over the word for probably hours. It's not a word you'd think would come up very often, but for some reason it does. Again, dialogue is different. It's easier for me to have a character say pants instead, but if it's something I'm writing as say a description of someone, I feel a bit silly saying pants because to me that means underwear.
I get around it by making things more clear. I'll often write 'camouflage pants' or 'combat pants' or decide that a character is wearing jeans.
I've called the story I've just posted 'Vacation' because in the US the word holiday is more likely to refer to a public or religious holiday than a recreational trip. You might take a holiday from work and use the time to go on vacation, but to go on holiday just sounds too damn English. Or maybe I'm wrong. That happens all the time when I'm writing. I type something, then I look at it and think, "wait, is that right?" then I head to Google and search for it.
I once wasted a whole night while writing 'Stay' because I wanted to use the word 'deckchairs'. It didn't look American to me, and Google revealed that it is a word used for a chair that sits on a deck outside a house. But whether is also means deckchair I couldn't find out. Nor could I find an equivalent word that I liked. In the end, I think I went for 'folding chairs, or something like that.
Anyway, I'm not sure why I'm writing all this. Justifying anything that looks a bit strange in my writing, I suppose.