Perhaps she uses both names to show that Janice is not comfortably accepted by the other students. Her size and rough personality did not lend itself to wanting to get to know her. When a person is referred to by just their first name, it sounds like the other person knows them well and as a friend. Janice doesn't seem to be friend material, at least to the younger students. Janice is a sad character that Leslie opens up to the reader. Leslie is the only one to call her by just her first name after she discovers that Janice's father is physically abusive. Janice must find some power in her life that is out of control at home. So, she uses her size and anger against those less powerful than she. Her friends are also called by their whole name, so maybe the younger students refer to them with full names as a sign of respect for those older than they are. It is hard to tell what an author intends unless you have the opportunity to ask them. Some authors have their own websites and respond to questions such as this. You might want to check it out.