Emily Wallace will be dancing off the wall at Washington and Lee University tomorrow, but it’s no college prank. The 22-year-old senior from Roanoke is one of 12 students who will do leaps, spins and flips off the side of a 40-foot building as the culmination of their aerial dance class. Their performances, at 3:30 p.m. tomorrow and Saturday on the exterior of Wilson Hall on the Lexington campus, will demonstrate “a completely new form of dance,“ said Wallace, an art history major with a dance background.