In Rodney Cline's 1960 paper, "The Conservatism of Philosophical and Educational Liberals," published in the Peabody Journal of Education, he declares that "Philosophical liberalism and Educational liberalism have strongly held sway in American Education for the greater part of the twentieth century."

Edmund Fawcett's book Liberalism: The Life of an Idea, traces modern liberalism (we'll define it here as preferring new ideas to the tried-and-true, an alternative to don't-fix-it-unless-its-broke) to the 1830s and the ideas of Wilhelm von Humbolt, a German scholar and diplomat who encouraged universal education.

There's an argument to be made that higher education became liberal once the notion of universal education became popular. In the 19th century, universal education was a radical concept in Western Europe. It had its strongest supporters in Germany, and many of those supporters migrated to the United States in the 1840s and 1850s, part of a flood of immigration about the time of the failed 1848 general European revolution.

Because conservative factions won that revolution almost universally across Europe, it was the liberals who emigrated to the United States, carrying their ideas with them. They believed in things like kindergarten (a German word, mind you), universal education, the elimination of slavery, and other radical concepts along those lines. German and Austrian immigrants made up an even larger portion of the Union armies in the Civil War than did the famed Irish regiments.

With conservative Democrats out of Congress because of the war, a liberal Republican-led Congress in 1862 passed the Land-Grant College Act of 1862, which provided grants of land to states across the United States to finance universities specializing in "agriculture and the mechanic arts."

This was a huge innovation, and it's a key sign that higher education was seen as politically liberal even in 1862. Indeed, President Lincoln was already leaning upon the politically liberal ideas of college professors when he approved guidelines for the treatment of prisoners of war and general rules of war to be followed by the Union armies in combat.

The Land-Grant College Act was a key milestone in the advancement of higher education in the United States, because it democratized it, allowing it to be accessible to many more Americans than before. Until 1862, colleges tended to cater toward the elite. Institutions like Yale and Harvard, among the oldest in the United States, produced theologians and lawyers, people among the upper strata of society.

They were exactly the people most inclined to want to keep society the way it was, because they had the most to lose if it changed.

The establishment of West Point was a step toward the democratization of higher education in the United States, because it didn't distinguish based on the ability to pay for the education. Furthermore, it taught engineering, which at the time was considered a "lesser" study than the law. West Point (and Annapolis, later) still had a high bar for entry, but it was a political bar rather than a financial bar. One had to be recommended to the institution, and while that was easier, it wasn't easy.

Once the Land-Grant College Act arrived, there were a wave of institutions to choose from, and competition begat lower prices in a classic case of supply and demand. Once World War II arrived, the Montgomery GI Bill ensured that anyone (Note: blacks were excluded) could attend these colleges, which by 1944 had 80 years to become established. That was the last step needed for the democratization of higher education.

Now, it should be noted that the whole idea of public education, enshrined in the United States from its earliest days (I'm thinking in particular of the Land Ordinance of 1785) is at its heart a liberal idea. The notion of giving a schoolroom education to everyone was a departure from the previous process of education through apprenticeships and journeymen in a trade. Previously, in Western Europe, general education was usually restricted to the elite, or religious institutions. Overton Taylor's "Liberal Education and Liberalism," published in the Jan. 1945 issue of Ethics, is a good walkthrough of how the idea of widespread lower education was also a liberal idea, but that's outside the scope of your question. I think it's worth mentioning, because it plays into the subsequent idea of higher education being a liberal force.

Note: This is a badly worded question, and trying to answer it is like trying to eat soup from a cardboard box.

Can you be more specific?

Are you asking when people started stereotyping higher education as liberal in nature, or are you making the assumption that it's de facto liberal now and are looking for a point in history when (and if) it shifted to more liberal values?

In order to have some context for the word 'Liberal' in this, would positivism be a liberal thing according to American standards?

If you're interested in the history of Liberalism in the West I'd recommend Eric Hobsbawm's Age of series.