What is the case against seeing the Medieval Catholic Church as oppressive?

The Catholic Church—since at least the time of Charlemagne, his tutor Alcuin of York, et al.—promoted learning, established parochial schools, and laid the foundations of the modern university. The Church is "the pillar and ground of truth" (1 Tim. 3:15).

If the Church were obscurantist, then why do so many cathedrals have such beautiful artwork, which very effectively instructed illiterate and literate people alike in the truths of Holy Scripture?

It is also a myth that only the clergy had access to Holy Scripture. Although wealthy medievals certainly had more elaborate Books of Hours, some poorer literate medievals carried around simpler ones, too, reading them throughout the day. See this video and this one (and these, too) for an introduction to Books of Hours and this website for pictures of medieval Books of Hours.

The following is an excellent historical work which overviews the medieval university, preparatory schools, popular education, technical education, women, hospitals, law, and economics:

Walsh, James Joseph. The Thirteenth, Greatest of Centuries, 1907.

In addition to spreading truth, the printing press also made it easier to spread error, untruths, and heresy. This certainly concerned the Church, and the Church's love for seeking truth and defending it prompted the establishment of the Inquisition (which did not execute a single heretic; the State, however, did).

The answer is Europe and the Faith by Hilaire Belloc.

He consistently and logically argues that the fallacy that you bring up is a result of collusion between the the North Germans who think their ideas and philosophy are better than everyone elses and the English aristocracy who found it necessary (until WWI) to have a close philosophical and spiritual alliance with the Germans to keep down popular uprisings of Catholicism within their nation (It was really quite dead by then).

He further says that because they (the Germans, or Teutons) were so late in the game to join with the Catholic Church, that the faith had no sticking power with them and they were the first to abandon it. So, because they like to claim (falsely) that they were the first to translate the Bible into the vernacular, and that that somehow brought forth enlightenment in to the world.

The truth is, the Dark ages were dark because there was a lot of reorganization going on. The Middle ages, on the contrary, were not the dark ages, they may not have been pleasant, but there was a lot less slavery and a lot more feasting during that time period. The dark ages were the period in which Catholicism defeated barbarism and the middle ages were when we remembered the antiquities, the renaissance, reformation and enlightenment (and all periods until the 20th century, where Plato and Aristotle are finally getting back in fashion [according to the Collected works of Aristotle]), were all reactions against the collective wisdom and tradition of mankind.

That, sir, is what your professor fails to profess, he thinks the greatest good is progress. Catholicism teaches that the greatest good is God and that God can only be truly known through Tradition.

I think, as is often the case, that in some times and places, the Church was co-opted by the Aristocracy. While the Church certainly had a vested interest in promoting learning, the Aristocracy did not. After all, Scripture is a revolutionary thing, and if the working classes (peasantry) became to well versed in exactly what the Bible teaches, they might be harder to control, and indeed even revolt. And given that the hierarchy of the medieval Church was often drawn from the Aristocracy, the notion that at least some elements in the Catholic church were oppressive, and attempted to keep some of the people from being educated, is at least a question worth pursuing. This is not to suggest, by the way, that all members of the Hierarchy of the church felt this way, but unquestionably some did.

On the other hand, the church did not always side with the aristocracy. In the New World, for example, the aristocracy as a whole sought from Rome a decree that the native peoples that they found here were not rational beings, as this would allow the locals to be put into slavery. However, the church refused, to the consternation of some of the Aristocracy in the new world.

I think this is yet another instance where the answer does not lie at either extreme, but somewhere in the middle. I would say, that in the long term, yes the Church did promote learning, and preserve the body of knowledge. However, in some times and in some places, it was less noble in this regard than in others.