I see you are under 25... your best bet is Council Travel they issue student fares, they allow you to buy 1 way fares without paying alot more, they are very flexiable tickets and esay to change. tehy are the american arm of uSIT travel in europe that handels under 25 travel... They are great. When I was going to school in France, they USIT took my student ticekt from CDG-WAW-EWR and exchanged it for CDG-IAD-ATL (at the time the plan was to non rev out of EWR-ATL) but I realzied how much i would hate my self going from cdg-waw-ewr-atl) and asked them if they would change the ticket... i paid like 100ff to change the ticket over.. nto a bad deal have them check into Lille though I wouldnt be surpised if the fare is the same....
their webpage is www.counciltravel.com
let me know how it turns out