Florence Robson, Aug 4th 2015, 14:50 Verified Guru
I don't have any personal experience of this (my year abroad was in Spain), but I've done a little research for you and discovered this page on the French social security system.
As you'll see, under the Employee section it implies that you don't actually need to be registered for a French social security number. As an EU citizen, you'll be covered by your EHIC for the first year of employment.
It's worth double checking this of course!