Lily came in with the reject wave, along with Josy. That means they started out a little behind the rest of the group who were there in class from day 1. Lily works regular hours outside of school, plus was caught up in the HOTs scavenger hunt and other activities so may not have been studying very much and got further behind than Josy. Lily had the opportunity to meet Burke at the preps party, so per dalli_x's theory she may have gotten test answers from him using her feminine wiles, whether they were the ones in the bathroom or some other set he had access to. Now that midterms are over she may not want to have to put out for Burke anymore and wants to actually pass her classes on her own as she's going into a demanding medical field where you have to know your stuff, and since Maya and Josy are now her sorority sisters she can ask them for help.