Before this happens, Edward explains to Bella that he worries about what might happen between them.  This is why his family is cautious of the relationship, too:  if things go badly, and Edward can't control himself around Bella, then they will all be in danger of being exposed.  If people know that Bella and Edward are going to be alone together, and Bella disappears (because he kills her), then Edward will be the likely suspect and his whole family will have to flee.

Edward tells Bella this because he wants her to be careful around him, and he wants her to be sure that people know that they are going to be together, to keep as a backup plan and to keep him honest.  But Bella goes in the other direction, lying to people about spending time with him alone so that no one will suspect him if anything happens to her.  She lies to Mike to tell him that she is staying home to study and do laundry the weekend of the dance, and Edward will be out of town.  Then she tells Jessica that her plans with Edward have been cancelled, too.  She calls Jessica so that there will be more than one witness to say that Bella had no plans with Edward that weekend, in case something goes wrong.  In a strange way, she's giving him a way to escape in case he murders her.