There are a number of turns of phrase used throughout the series, largely Americanisations, which didn't come into use in the UK until much later; Referring to individuals as "dude", referring to a mixed-sex group as "guys", terms such as "I'm Good", "hot mess" etc. I would assume the blame for this lies with the young cast members being allowed some element of improvisation whilst being unaware of the linguistic norms of the period.