When I visited Yale I was in a group of about 20 prospective students. I didn't know that I would be in a group where about 1/2 of the people were there to interview and the other half just for the activities.
I wore dress pants and a light blue button down shirt underneath a navy blue v-neck sweater. (Had to go keep with Yale's color scheme

) I felt properly dressed given my reason for being there. I didn't feel under-dressed standing next to those who interviewed and were in suits, and I didn't feel overdressed standing next to people in just shirts and ties. Other students who were in the group not there for an interview wore dress pants & ties. Girls wore knee length skirts, WITH STOCKINGS/LEGGINGS, heels - didn't see any flats. Some girls were in suits with button down shirts underneath, but be careful with how many buttons you decide to unbutton. I thought a couple were inappropriate.
I wouldn't recommend wearing jeans with a shirt and tie... it looks weird normally, let alone on a b-school visit. Polish your shoes too... it adds an extra touch!
Which school(s) will you be visiting?