Dining in Paris for group of 12 including teens

Brasseries like La Coupole and Bofingers are a good choice for groups - plus since they serve longer hours ( many restaurants do not even open before 7:30 ) they are good for families who like to eat a bit earlier .
Not outdoor cafes was they have history and atmosphere.
Do make reservations - impossible to wander into almost anywhere with a group that size .

Hopefully you will not all be joined at the hip and can take the kids and sit at outdoor cafes for lunches or late afternoon drinks , fun to people watch !

Take them to Bouillion Chartier; they will love the atmosphere and it has a big menu with lots of choices. (yeah, not fine dining, but you have a group of teens so maybe not the primary thing). The waiters look like they stepped out of a movie the room is atmospheric and they tote up the bill directl on the table cover (l'addition -- this is where the term for a dining bill in France comes from, this old tradition of writing the order on the table cover and adding it up right there). It is large and has long tables; I would think they could easily accommodate a big group and it is cheap and takes reservations. Lots of choices of classic French foods and a nice selection of desserts for low cost.

https://www.bouillon-chartier.com/en/menu

One dining tip I have is to encourage your group to have fun dining in Paris, but lower their voices in any restaurant. It may be hard with a large group and with teens, but a lot of loud talking or laughing will be disruptive. I have seen French teens be loud many times --but it's usually outdoors or perhaps on the metro. In restaurants and cafes the French talk quietly.

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If any kids (or adults!) are Harry Potter fans, the residence of Nicolas Flamel (Dumbledore's friend and actual historical figure) is now a restaurant - in the 3rd arrond.
http://www.auberge-nicolas-flamel.fr/histoire.php
It may be a bit "posh" but our kids loved being there and rose to the occasion.
I made the reservation on lafourchette.com - they seated the 12 of us (2 families and friends) in an upstairs loft all to ourselves!

Have fun!

BGs tip is a very good one. It’s considered rude to be loud anywhere in France, but especially in a café, bistro, or restaurant.

No one has mentioned Polidor in the 5th. Oldest restaurant in Paris, in most guidebooks, communal seating at long tables.