BOE 14 or 21 days with teenagers

Those in their mid teens most often like a little independence. I don't know that they would like the structure of being on a bus with a bunch of "old folks" that long. You might want to ask the teens how they'd like to travel? Have them research travel and present an itinerary they'd like to do, and specific places that interest them.
By all means, take time to enjoy Paris.

The 14-day variety might be better for your party. We took this itinerary with our 18 y/o and while it was a great experience, baked into the 'Best of' characteristic is constant moving from locale to locale. Witnessed a 'toll' in her energy/interest levels accruing over time, especially as the tour got close to its final destination (Rome). The 21-day RS tour would've been 'too much of a good thing' for our circumstance. That said, each child is different and you know your two better than anyone. One idea you might want to consider, which is what we did, is to arrive/spend a few days ahead of the tour by yourselves (get acclimated to timezone, weather, and see stuff not included in tour) and add a few days after its conclusion. This way you still maximize three weeks in Europe, but enjoy a more family-friendly pace at the beginning and at the end.

I agree with Matt. I have done the 21 day tour, solo not with kids and there were no kids on my tour date. It was pretty strenuous. The long days in Amsterdam and Rome they could handle, but the constant change every 2 days (or every day in 2 instances) is very wearying and would probably be too much for many kids (and adults, lol). I agree that you might consider adding time at the beginning and end. I would do at least 3 nights ahead just to get adjusted and over jet lag. I would add a couple of days at the end as well, wherever you end up.

Are you thinking about the family one that starts in Amsterdam or London or are you thinking of doing the regular one that starts in Paris?

Family Europe: London to Florence in 13 Days Tour - could be followed up with Rome and the Amalfi Coast 'on your own'. http://www.ricksteves.com/tours/europe/family-europe-london-florence

Family Europe: Amsterdam to Rome in 14 Days Tour - could be preceded with London and Paris "on your own'.
http://www.ricksteves.com/tours/europe/family-europe-amsterdam-rome

I took my daughter on the RS Family Europe tour, she loved it!

In our tour group we had 26 people. Of those, 14 people were aged between 10 and 17. The kids all had a ball, they took over the back of the bus, they hung out together , and they were overall a fantastic group!
I recommend you consider the Family tours.

Of course you should spend a few more days in Paris, our tour started in Rome , we arrived there 2 days early, and tour ended in Paris, where we stayed an extra week. Many folks tacked on extra days pre and post tour, I felt sorry for those going straight home after tour, Paris definately needs more then the two tour days!

Thank you so much, everyone! Your comments were so helpful! I had been thinking the family tours were for younger kids, but if there's more teenagers, then it would be fun. I will look into those and also really valued the input of staying more before and after the tour. Thank you!!!

" I will look into those and also really valued the input of staying more before and after the tour. "

Well, what you do need to know about advice on this forum is that everyone loves to travel so NO ONE will recommend you spend LESS time in Europe, lol!!