Au Pair in France OFII question [closed]

I am living in France (have been here just about 2 months) and have an au pair contract that runs from September 15 2013 until August 2014 (long stay visa). I have the visa in my passport, but it just became apparent to me that I am missing one step: the medical appointment with the OFII (French Office for Immigration and Integration) and my visa stamp validating my long stay visa. When I realized this, it came as blessing more than an "oh no" moment, as the au pair experience with this family is really not how they presented it to me, and I am feeling very ready to go back home to the USA. Here is my question: if I simply ignore this OFII paperwork and wait for my 90 days to come to a close, will my departure from the country be necessary as my long stay visa no longer be valid, or will my host family simply call the OFII and get it sorted out? Note that I will be bringing this to my family's attention AFTER I have been in the country 90 days. I'm just curious if my long stay visa will legally be voided at that point, or if it's an easy fix even after my 90 days have ended.