Interesting... In Quebec (a province in Canada), we have CEGEP, of which the 'C' is "College," and the purpose and position of it seems fairly similar to that of Russian College.
For reference, Collège d'Éducation Générale Et Professionnelle, but we can't call it GAPEC in English...
College student should be accepted. But also you cant put an execise to "choose the words" where the only translation offered for "university student" is "university undergraduate". Please think of that. You become a university undergraduate even not been a university student. It is enough with finishing high school.
The use of 'an' comes before a vowel sound, not necessarily a vowel letter. A couple of examples: 'an hour' because hour starts with an 'o' sound, 'a uniform' because uniform - like university - starts with a 'y' sound. Then there are the oddballs like 'historic' or 'Hispanic' where you may see either a or an depending on how the word is pronounced. Hope this helps!