Where did medieval university students have to go to school before they could attend university? Today, there's K-12 education and other national education systems. But how did it work in the medieval age? Did students just go to university without any prior schooling (perhaps knowing how to read and write), or was there a sort of admissions process or requirement for prior knowledge?
There are several issues to unpack in this question.
Firstly, it should be confirmed that, yes, prior instruction was required in order to enroll at a medieval university. This being said, the only real requirement for the baccalaureate of arts was a working knowledge of Latin, in which all lectures were delivered and all texts written (unless we're talking about "language courses," obviously). Because lower education would not be standardized, let alone socialized, for several centuries, how the prospective student came by his knowledge of Latin really didn't matter. Those that could afford it were privately tutored (more common towards the end of the Middle Ages, and even then most common in Italy and Germany, where there was a higher percentage of aristocratic students), but there were also Latin schools that operated alongside the universities.
Where the higher faculties (medicine, law, and theology) were concerned, one usually had to be a Master of Arts, though exceptions would be made in a number of cases, including (again) if one had prior private tutoring. There were also the mendicant schools, where members of the Franciscans and Dominicans (primarily) were instructed in arts and theology. These schools are somewhat problematic, in that they are sometimes considered interchangeable with universities, and sometimes not. For the purposes of this question, members of the mendicant orders received a comparable university education prior to embarking upon theological training, but did not take arts degrees.
Secondly, there is also the matter of age. While university is today primarily the province of young adults, it would not be uncommon in the Middle Ages to see young teenagers, particularly in the arts courses, at universities throughout Europe (particularly in the Anglo-French tradition). At the aforementioned Latin schools, I've seen records of children as young as eight years old (Ave Maria College, Paris). This didn't leave a lot of pre-university time in which to undertake study.
As far as I know, there are no existing accounts of / records from the medieval application process. It is most likely that a student went to the city in which they wanted to study, and applied directly to masters until they were either accepted or gave up.
For more information, I would recommend taking a look at de Ridder-Symoens, Hilde, ed. A History of the University in Europe. Vol. I: Universities in the Middle Ages. Cambridge: Cambridge University Press, 1992.
Feel free to ask follow-up questions.