The content of our site is written in American English—color, honor, valor, capitalize, "punctuation inside quotes," and so on.
With respect to spelling, we don't specifically test anything that is different in American and British English; the site will never require a student to type a word that differs between American English and British English spelling. The exercises on using punctuation with quotes are the only ones that might be confusing for your students; you could always just steer clear of those. We usually defer to Chicago Manual of Style and/or the Modern Language Association on controversial matters.