I'm a teacher for my day job, and I work with teenagers.
YES, the girls have to go to the bathroom ALL THE TIME. They sometimes ask multiple times in a 50-minute class.
YES, they always try to go in groups.
YES, they have told me that usually they don't actually have to use the toilet when they ask to go to the bathroom.
Who knows what they actually do in the bathroom! It's a big mystery. But it's not my business. Under most circumstances I let them go freely, one at a time, as needed during class. Outside of class, they always go in groups of at least two.
To address the OP directly: I recently chaperoned a group of twenty-four teenagers on a trip overseas. The girls were out of their seats throughout the flights, wandering the cabin in small groups, sitting five or six across in a middle row of four seats (their hips are small enough that they can do that), laying across each others' laps to take a nap or watch a movie, etc. Thankfully, we had a block of seats to ourselves in the middle of the cabin, or else those girls would have been quite a nuisance to other passengers.
So, while I cannot begin to fathom why the girls seek to go to the bathroom so much, I can understand why they always need to be out of their seats. I think it's just part of being a kid--restlessness, etc.
But good for reducing the risk of DVTs. I try to stand up and walk around every couple of hours on long-haul flights, and I also do some stretches in the galley or lav to the extent that I can. During layovers I almost NEVER sit down; I just keep walking around in the terminals/lounges. If there is a workout room, all the better! Staying mobile is very important during travel!