As an FYI - it may be better to report mistakes with the button in the question itself, as it gets to the course creators that way. That said, there is often little response from that, and a general discussion can be good.
In the USA we use "ciao" for both hello and bye (the same in Italy?), but I think it's more informal than "arrivederci," which may be more like "good bye" or "farewell."
I would not say Arrivederci is formal. You do not say (at least you are supposed not to) Arrivederci -for instance- to a professor you do not know at the university. You could say ArrviderLa, Professore. Or Buona sera Professore. Or Conto di rivederla il mese prossimo, Professore, and so on.
When you get in, if they are colleagues you call by their christian name, you can say either ciao or buongiorno a tutti else, if there's your boss or your boss' boss you would rather say Buon giorno dottor...Rossi...; ingegner....Bianchi.., signor..., Neri...; Come sta dottor Verdi? Or Buon giorno a tutti. When you leave you could say buon giorno again, or ciao following the circumstances. Leaving you could say -as well- arrivederci, if it is expected that you could meet again, but it could sound a bit unofficial or informal if the context is supposed to be formal.