Your question hits on regional differences. To my ear, "university" sounds wrong when preceded by a preposition such as "in university" or "at university". I cannot explain why this is, because the similar words "school" and "college" sound fine to me when preceded by a preposition: "in school" or "in college". I speak U.S. English. In Britain, or perhaps in other parts of the U.S., "in university" might sound fine. I don't know. For it to sound right to my ear, I would need to say: "Students decide to take a job while in (or "at") a (or "the") university ..." but you will be understood no matter how you say it. It's a question of usage, not correctness.

Hi Jane I would simplify it by saying ..... "University students usually take a job for one of two reasons." Then you can give the reasons. Example: "Firstly(first), they need some sort of income while studying and secondly(second) they are trying to get some work experience." Or whatever your two reasons are. I hope this was helpful.