This visit is almost the equivalent of going to a med school's open house, which can be very useful and informative. I went to one medical school's open house for 4 years straight, and it ultimately was the school I ended up going to.
Dress professionally. You don't need a suit, but business causal would be nice. Do some research on the school, and ask any questions you have about the school. Try to get a sense of if you would like the school. You are not under the stress of an interview and being evaluated, so enjoy the experience and ask yourself if you would be happy going there for 4 years. Talk to the students and see what they have to say.
I did this at the school I am currently at and those school visits really allowed me to vet the school and decide if I wanted to go there.