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"?
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