I have heard of a number of yeshivot that have a strict rule regarding boys and girls being with each other. Their rule is "simple" - outside of your own family members, it should be never for any reason.
Let's say Reuven's friend, Benzi (who are in the same class) has a sister, Talia. The three of them walk together on the street on Shabbat to visit their friends, Habakuk & Reut. Is this activity "marit ayin"? Keep in mind, that there is no prohibition, per se, even according to the yeshiva's rules about Benzi talking to Talia, as they are siblings. But, people don't know Talia, who don't know the family. Also, it IS against the yeshiva rules for Reuven to be with Talia.