I've also done this trip but was a solo traveler. I did it in a September so during the school year and no teens on it.
The bus trips are broken up by stops for various things. For instance, Day 4 lists "Bus 5 hours". Well, it's 2h45m from Paris to Guedelon where you'll tour the newly built castle, have talks from various artisans and have lunch. Then you'll ride another 1h 45m or so to Bourges for the overnight.
The accommodations on Mont-Saint-Michel are varied. Tour members are divided between a few different locations. My solo room was OK for one night. Will your teens be sharing a room? You might want to talk with the RS office about making sure you all are in the same building or actually this is a very safe-feeling location so it would probably be OK for them to be housed in a different building.
It IS nice to get out of the city and you will see a lot of interesting countryside on this trip.
The only thing I would worry about in July is heat unless you all are tolerant of warm temps. I am not.
I'll add:
I'd have the family watch the old black and white film The Longest Day. It runs on Turner Classics from time to time and is also available on Youtube. The day you tour the Normandy landings will be an important history lesson for them.
IF by the time you get to Giverny you and your kids are very interested in WWII history I'd recommend a walk down the main street of Giverny to the church where Monet is buried. In the churchyard is a memorial to a Lancaster Bomber crew that went down on the night of June 7/8 near Giverny with all on board killed. In this beautiful peaceful looking village it's a shock to realize this was in the center of fighting in June 1944.
IF you can go a few days early to Paris I would. Even if you've been there before there is SO much to see!