If your interview is not over, you need to think positive and focus on your assets. Sounds like your work history is one of them. Don't waste your time whining about not being in the top third of your class. The way you make it sound, you are probably in the top half. That being said, you do have a shot. Lots of firms hire from the top 1/2 after 1L OCI.
Firms do waste money on screening interviews, a lot of times just to be like==I interviewed such and such number of people for this job. So, I wouldn't exactly agree with the notion that firms only interview people they are "serious" about. If its an interview at the firm, with lunch--they are thinking seriously about you. If its just an interview and they don't tell you what type (or won't tell you what type), then more likely than not, its a screening interview. Always try to find out what type of interview you are doing--that's a hint as to how you should prepare.