I think it was set in Paris. A group of teens hung out in the sewers. I think some tourists ran into them and were killed. The only concrete thing I remember is the last scene. A woman was crawling out of the sewer in the side of a hill. It looks like she escapes. But then she is dragged back inside.
Murdering teens in the sewers
→ in Movie Questions
The description of the final scene made me think of the closing stages of Ils [Them], set in Romania not France though and probably not what you are looking for anyway but might as well throw it out here just in case.