"This is the classmate of whom I ´m so proud." and:
"This is the classmate who/whom I´m so proud of" ??.
I mean, can I always choose to put the conjunction(in this kind of constructions) at the end of the sentence or before the object??. I would bet I usually come across both ways interchangeably.
We're in a couple of gray areas here. "Of" is a preposition. There used to be people opposed to placing it at the end, but it's okay these days.
If the pronoun (who/whom) is object of the preposition, some would say it has to be objective case; but sticking it at the end seems to eliminate the problem, and "whom" is in disfavor lately.