Basically, what happened is that the Catholic Church in America as represented by the USCC and NCCB (which combined to form the USCCB a few years ago) decided that the door had been opened for them to make changes. They ignored the documents of Vatican II, to a large extent, and just did what they wanted. In my mind, they engaged in deconstruction and reconstruction – or to put it more kindly, they were far too “progressive.”
For example, Vatican II said that the Tabernacle could be moved to the side of the altar, rather than be directly behind it. The American Bishops moved it FAR to the side or even into an adjoining chapel which some churches have for use with daily Mass.
Vatican II opened the door to modern hymns but praised Gregorian chant and recommended keeping it in the liturgy – the American Bishops promptly threw out the Gregorian chant entirely.
Vatican II opened the door to replacing the Crucified Christ behind the altar with the Resurrected Christ – the American Bishops went so far as to allow naked crosses with no Christ at all (you would think you were in a Protestant church!).
Vatican II opened the door to using Eucharistic Ministers if needed in extreme cases (a LARGE parish with one priest) – the American Bishops instituted it in full for all parishes and all circumstances.
Vatican II opened the door to church architecture such that gothic and baroque styles, with gargoyles, etc. need no longer be the norm – the American Bishops began building post-modern churches and cathedrals that look like office buildings.
There are dozens of such examples. Vatican II “opened the door to let in some fresh air” and the American Bishops knocked down the walls as well. What happened to the Catholic Church was nothing short of tragedy! I do not blame Vatican II – I blame the wildly progressive American hierarchy for these things. They were given an inch, and they took a mile. This, I suspect, is what your theology teacher is trying to say.