If Tara moved back on campus,the Dean of Housing would have assigned her to whatever vacant room was available.It's not unusual for a student to move off campus, whether to private residency or just dropping out, leaving her former roommate alone in a double occupancy room. (Given the mortality rate around Sunnydale, I doubt that there would be a shortage of available housing) Obviously, that is what happened to Tara. She simply moved into an available room, whether shared by another student, or by herself (Did we ever see Tara with a new roommate? I can't remember)