A couple doesn’t need to make their relationship public; lots of people keep it a secret for weeks to see if it’s working before even telling their closest friends.
Furthermore, French teenagers keep their relationships very private.
The boyfriend/girlfriend gets introduced to the family only if the couple has been dating for a few months, and we usually don’t talk about our love life with our parents.
When you kiss at a party, things are a little bit different: You might need to talk about what happened the night before and decide if you want to be an item or not.
Well, we usually go out in groups and meet within this social group. If you are already friends with the guy, you just spend more time together, get a coffee after school or share a meal at your apartment, and flirt a little bit.
If you just met at a party, well, you kiss, and things evolve naturally.