The students at Early Modern universities might be a mix of nobles and commoners, but by the 16th century universities and colleges were traditionally associated with students who were decidedly not nobility. When noblemen attended, they were lucrative interlopers. You get men like Thomas Starkey in 1529 describing the English university as the place "poor men" sent their sons to be educated, even as he sought to outline the qualities that noblemen's education should have for the enrichment of the upper classes:
lykewyse as we have in our unyversytes collegys & commyn placys to nurysch the chyldur of pore men in letturys, whereby as you see commyth no smal profyt to the commyn wele, so much more we schold have, as hyt were certayn placys appoyntyd for the bryngyng up togyddur of the nobylyte [...]
Universities and colleges were institutions "to nourish the children of poor men in letters". Later in the 16th century, you get English writers complaining that the presence of noblemen's sons at universities diluted their original purpose to the point where it became difficult or impossible for "poor men's sons" to get the education they came for. Noblemen's sons might come to college to drink, fight, and fool around, to get as much of an education as they personally desired and no more; this state of affairs was perceived by commentators sympathetic to common-born university students as lowering the tenor of the establishment for anyone actually seeking a strenuous education. On the flipside, authors writing for gentlemen students attending university advised them not to mingle outside their own social class, not to be overly free with one's social inferiors. The possibility of inter-class mixing and inter-class envy between nobles and commoners was real, but the anxiety these possibilities provoked might have been a little inflated even in the era of town-and-gown conflict.
What were the incentives for young noblemen attending university, besides the knowledge you were somewhat less likely to get kicked out for drinking and fighting? As "fellow-commoners" they received perks like the right to dine at high table with the Fellows, they were largely exempted from actually attending lectures, and they didn't necessarily have the end-goal in mind of earning a degree; accordingly, many did not matriculate or obtain a degree. (For students of the lower classes, this would have been a devastating setback; for the sons of noblemen, not so much.) They could bring their own staff with them and obtain a substantially higher quality of life than students who were not of the upper class.
Even so, there was a lot of range in the category of students who were not noblemen's sons -- "poor men" need not actually be men who were poor, they could be financially successful in trade or wealthy gentry but simply not born into out-and-out nobility. "Common birth" didn't necessarily mean you showed up for your first day at university fresh off the farm with straw in your hair, totally unlettered -- Cardinal Wolsey was the son of a butcher, Christopher Marlowe the son of a shoemaker, both attended grammar school before they attended university and both had (wildly different) careers in letters where that university education came in handy. Walter Raleigh's financial struggles at Oxford kept him from matriculating, but not from later success. Discrepancies in class and financial security led to some pretty broad possibilities for students attending university -- if you were from a wealthy family, noble or not, you had more leverage and thus more disciplinary leeway vis-a-vis misbehavior, actually attending class, and so on. Furthermore, your social class and rank going in made a difference in how you were going to use your education afterward -- for employment or just for personal edification -- and thus impacted how seriously some students took their studies. Noblemen got by just fine without a university education, and noble university students knew it, but it was individuals of common birth for whom the university was their primary means to self-advancement.
Some other questions that might be relevant here for conceptualizing student life:
(For what it's worth, I think Shakespeare's treatment of Wittenberg owes a lot to his expectations about Early Modern English universities, though Shakespeare himself never attended one -- for one the fact that Horatio and Hamlet rubbed shoulders there across lines of class, Hamlet as a prince and Horatio as a poor scholar. Elizabeth Hanson's "Fellow Students: Hamlet, Horatio, and the Early Modern University" goes into this and the class dynamics, imagined as well as real, in English universities. Another famous Wittenberg grad of the Early Modern English stage is Marlowe's Doctor Faustus, born to parents "base of stock".)
For reading recs, James Hannam's bibliography for reading on medieval and Early Modern universities is great and I know I'll be plundering it the next time I write anything touching on this area.
I may only talk about early modern Spain, I hope you get answers concerning other places.
The Spanish universities of the XV and XVI century mostly granted degrees in Theology, Civil Law, and Canon Law. This circumstance tended to bias the sutdent pool towards potential lawyers/attorneys/notaries and potential bishops or high clergy. There could very well be an overlap between the two, as the high clergy would normally have studies in Canon Law, considering that bishops would have temporal power with civil and criminal jurisdcition, having to act as judges and notaries when circumstances called for it. The Inquisition too functioned as a Court, so it had to have jurists specialised in inquisitorial procedures and Canon Law.
Amongst the students in Spain, you could find mainly two classes: people rich enough to afford to send their kids to university in the hopes of them getting to have a high-level lawyer, maybe a member of Royal Council, or a President of the Royal Audience of Justice of Valladolid or Ganada; or crazy smart students that would receive a stipend from either a college or a bishopric.
Under the first category you normally find hidalgos (low nobility) like Hernán Cortés, who studied Law in Salamanca for a couple of years, or Juan de Arce de Otálora, who studied Law in Valladolid and managed to be President of the Royal Audience of Valladolid. You could also find sons or nephews of bishops hoping to get their father's or uncle's office, like the cases of Diego de Muros III (nephew of bishop Diego de Muros II), Fernando de Valdés Salas, or Alfonso de Fonseca III. Curiously enough, these three people created universities of their own: Diego and Alonso established the University of Santiago de Compostela, and Valdés Salas created the University of Oviedo.
Spanish universities received a big impulse in the times of the Catholic Monarchs, as they promoted a civil servant caste to some of the highest ranks in the land, creating a modern judiciary system. Viceroyalties also needed civil servants, judges, the lot, so the universities were more necessary than ever.
Urban patricians fell under the same category as hidalgos, with the hopes of getting ennobled through relevant services to the Crown in the field of Law. In some cases prior to Philip II, rich enough patricians may have sent their sons to study abroad, like Juan de Enzinas sending his sons Diego and Francisco to study in Louvain, very close to the Antwerp and Ghent hubs.
There were some cases of the opposite: italian patricians sending their sons to spanish universities, as was the case with Girolamo della Sommaia, student in Salamanca (1603-1607).
As for the poorer fellows, they would normally receive burses from colleges or from important patrons, sometimes doing menial jobs for richer students, including literal benchwarming. These poorer folks would normally graduate in order to become lawyers and maybe climb a bit the social ladder, or go into priesthood.
Sources:
Enzinas, Francisco de, Memorias, translation from the latin original by Francisco Socas. Madrid: Ediciones Clásicas, 1995.
Enzinas, Francisco de, Epistolario, translation and edition by Ignacio García Pinilla. Geneva: Librairie Droz, 1995.
Andrés Ordax, Salvador, Santa Cruz, arte e iconografía. El cardenal Mendoza, el colegio, y los colegiales. Valladolid: Universidad de Valladolid, 2005.
Sobaler Seco, Ángeles, Catálogo de colegiales del Colegio Mayor Santa Cruz (1484-1786). Valladolid: Universidad de Valladolid, 2000.
Can you define pre-modern? Do you mean medieval, or pre-French Revolution?