Rules Of American English

[Kött]

At the moment I am in the middle of an internship at school (as I am studying to become a teacher) and I will be teaching a 7th grade for a couple of lessons.
In one lesson, I will be teaching them the basic differences of British English and American English. As some of you may know, German kids learn the British variety at school (however, their English usually quickly changes after a couple of years; if they actually use it that is...)

Anyway, in order to introduce the topic, I have come across a couple of basic rules for writing, such as "words with "our" are changed to "or" in AE".
All of these rules are pretty clear and please don't get me wrong... I know that the way you spell words doesn't really matter that much to any native speaker and that Amarican people also know typically British vocab... but I gotta teach this stuff so it better be correct...
So, there was one rule that didn't make much sense to me at all... or I never really paid that much attention to it...
It's the following one:

Words containing a double consonant before ING and ED have been changed to one consonant (see travelling)    (This American spelling is becoming acceptable in the UK)

What? Acceptable in the UK? Shouldn't it rather be the other way around? I mean "traveling" looks kinda familiar but I've never come across a word like "prefered".
Also they forgot to mention that IF it works for ING and ED, it should work for ER after a double consonant as well...

So something must be wrong with that line... right?
Can you guys tell me when exacly that rule applies and when it does not apply? And if you have to stick to the AE variety (as for instance in a CV) would you actually have to write "traveled" instead of "travelled"?

[MarcK]

I remember the word traveling a lot more than travelling, however I've never heard of the word beging. I think it's really just specific to the word, and how reading some words under this word would sound awkward as fuck, like barring vs baring. English isn't a really good language to have really general rules like that.
also in response to the "ER" thing I think that wouldn't qualify as a general rule because there are words like mother. For verbs only I guess it'd work because of words like Spot/Spotted/Spotter, but to me, Spoted and Spoter sound like wildly different words when I read them.

[C-G-B]

do yourself a favour, zip your back up get your head on straight get some drinks and head on over to the english club.
http://www.englishclub.com/writing/spelling_add-ing.htm

It has to do with not the letters themselves but the syllables see. 

[C-G-B]

  oops

[Kött]

That answered my question perfectly well, thanks!