I voted for students, merely because a student will eventually graduate. I would also make it so that each year, the students get pushed into the next year level, so that they have a chance to graduate. This means that if you decide to focus on student characters, people still have a chance to have an adult character, they just have to work for it a little bit.
One of the other reasons students are appealing is because so much growth and turmoil happens while you're growing up. Quite apart from the dangers of evil, there are the dangers of growing up and making social faux pas, or not being popular enough, or any of the other things we all know about from personal experience. This means that on a student-focused site it's much easier to set up your character's own private torment even if they're not actively part of the main site plot.
It would also give the staff time to create a plot that doesn't focus on one or the other, but brings both groups together, which is what I think we should be aiming for if you want to have a site that welcomes both adult and student characters.