Would it be appropriate to use "you all" instead of just "you" when its plural? You could also use the Pittsburgh yinz :)
I don't see why you couldn't, it helps clarify that it's plural. When I took Latin in high school, the teacher strongly advocated the use of "y'all."
What is the significance of the "is" at the end of the word "studetis"?
The ending '-tis' is a second person plural ending. 'Where do you (more than one person) study?'
Is it incorrect to say "Vos ubi studetis?"