First impression is a lasting impression.
It depends on which school you are visiting. A rule of thumb might be to dress nice enough that people know you care about your appearance and the impression you make, but not to overdress, unless you are interviewing. Jeans and tennies are out, as is the evening dress.
You can dress better for urban schools than for schools in college towns. In NYC, L.A., S.F., Chicago, ATL, Dallas, Houston, Seattle, Philly, D.C., dress it up. You'll see some students who are dressed really nicely, so you won't look out-of-place.
In college towns, go for a nice pair of khaki capris or a nice slack with a pretty sweater or blouse and a nice shoe. You want to be dressed slightly better than the students who go there, so people know you're on business, but without appearing stuffy.
Be sure your hair, makeup and nails are immaculate no matter what. And carry a nice bag. Be organized, take notes. Go easy on the purfume or eschew it altogether.
The way the girl on the right in the photo above is dressed is appropriate also. She's hot. Another suggestion, look at "Gossip Girl" and go for the preppy look they often show. That works all over the place.
If admitted, you can dress down a little, but I would still look nice.
And also, whatever you do, DON'T follow this guy's advice (with the exception of the hair/nails looking kept well piece). If you every wear khaki capris, you'll forever be known as that weird matronly girl. My mom wears khaki capris...she's 65.