I grew up speaking British English. The words I learnt were occasionally marked off in papers, despite their being English words. Are words like betwixt, trebble, learnt acceptable in papers for English classes for professors in America, specifically Texas?
"Treble" (in British English at least) can mean the same as "triple", as well as high in (musical) pitch, as in the opposite of "bass" (see also the Wiktionary entry).
"Learnt" (again, British English) is an alternative past form of "learned" (Wiktionary again).
My $.02 USD:
Betwixt: not commonly used nationwide; I've heard it (and used it) in New England.
Trebble: We don't double up the b. It's spelled 'treble' and it is still in use.
In American English, we don't substitute the -ed ending on verbs with a t.
Learnt = learned Spelt = spelled
and so on.
Without wanting to speak ill of Texas... it's Texas. They have their own dialect of English down there.
As a bit of anecdotal evidence, my nickname is "Lin" (short for Linda). My Texas friends manage to take the letters Lin and somehow stretch two syllables out of them. "Le-in" it becomes. Why? I have NO idea.
Treble might be more acceptable if the US played darts more often. In BE, the 3x multiplier ring of a dart board is called the treble ring of which the highest scoring segment is the treble twenty. http://www.pdc.tv/staticFiles/b6/b3/0,,10180~177078,00.pdf