Undergraduate students are required to take a distribution of courses to satisfy the university's core curriculum known as the Common Core. In 2012-2013, the Core classes at Chicago were limited to 17 students, and are generally led by a full-time professor (as opposed to a teaching assistant). As of the 2013–2014 school year, 15 courses and demonstrated proficiency in a foreign language are required under the Core. Undergraduate courses at the University of Chicago are known for their demanding standards, heavy workload and academic difficulty; according to Uni in the USA, "Among the academic cream of American universities – Harvard, Yale, Princeton, MIT, and the University of Chicago – it is UChicago that can most convincingly claim to provide the most rigorous, intense learning experience."