I have arrived at a class, I have arrived for a class and late for a class. I have never arrived to a class, on time or otherwise, Yes, I have come to a class and for a class, but never at a class. I have shouted at a class (mea culpa), Why I wonder is "came" allowed for "chagaram". Is it to justify the use of "to" after "arrived"?
They only gave "arrival words", not came. So what is to be expected of the students responses anyway?
I think a literal translation would be "arrived at the hour", which would not be incorrect in English but is very stiff and formal. We would use "come" + at/on in everyday speech to convey this meaning. So while the translation is not direct it is probably the best way to communicate the meaning and tone of the portoguese sentence