Question: How diverse is the student game developer population in this school, overall? How about the top 10% or so?
What to look for: Ideally, you'd like to see a wide range of socioeconomic backgrounds and demographics, but in most cases you won't. At least shoot for more diverse than the game industry.
What to do: Schools generally know the demographics of their student body. They also know who the top students are. Not all schools put the two together to see how they overlap, so you might have to do some detective work on your own. Grades of individual students are confidential (as well they should be), but you can see if the Dean's List is public, and then take a guess based on names and any other information that happens to be there. When you visit campus (you are going to see the place for yourself before you commit to spending four years of your life there, aren't you?), you can also get qualitative information from existing students.
What to watch out for: If all of the students in the program look like they were all cloned from the same genetic material, it could mean several things. It could be that the school is actively selecting people that fit specific criteria, which could signal that they're more interested in being a factory that churns out degrees than actually caring about you as an individual. It could be that the school has difficulty attracting women and minorities, which means you'll be less sensitive to diversity issues than you should be if you're white/male/straight, and you'll be feeling slightly uncomfortable (at best!) if you're not. If the student body within game development is diverse as a whole, but the top students are all white/male/straight, then that suggests the program is set up to reward certain types of students -- likely because the faculty look like clones, even if the students aren't.
In general, a diverse population means that a wide variety of people can succeed at the school. Without it, the implication is that exactly one type of student succeeds, the one who can Fit In Here And Be Just Like Everyone Else. If you feel like you'll fit right in, this might be okay... but take a Women's Studies or Minority Studies course anyway, will ya?