I wrote the literal translation too, and I thought that it was acceptable to use that expression in English to mean "I was mistaken for a student", but then I realized I only thought that because I know the Spanish version of the expression and it resembles the Portuguese version. After reading these comments, I searched for "I was confused with a student" in Google - there were only two results, and this comment page was one of them.
I disagree. I admit I'm from western Canada though I think what I speak is similar enough to American English. I would interpret "confused with a student" to be "confused [about something] just like another student". However, I would say "confused as a student" could have the same meaning as "mistaken for a student" if there is proper context.
In the northwest US, (presumably just an hour or three south of you?), "confused with" would mean "mistaken for" unless context gives you other clues. "Confused as" would mean "confused when I was" unless context gives you other clues.
Admittedly "confused with" feels more context reliant, and more likely to mean "confused along with" when you put it in the passive construction.
Most Portuguese words are stressed on the penultimate (second last) syllable, with the exception of words that end in I, U, L, N, R or Z; or have a diacritic mark above a letter that indicates the stressed syllable.
Confundida does not contain any of those exceptions, therefore it is pronounced con-fun-DI-da.