Switching from school language to home language

I have been quite intrigued with my younger batch of kids. What intrigues me, is that they continue to communicate with each other in German--even though they all attend an English speaking school together. They even all go to ESL class together while at school. My older kids had switched to speaking English to each other by the time they were in school. Yet, these younger ones are simply more comfortable communicating in German. It's fascinating that as soon as they step off the bus, they switch from their English world to their German world. A few months ago, I videoed our after school conversations at the bus stop and on our walk home (see video below). It was fun to see how they refer to things at school by the English term: "gym" and "library". But they still describe and discuss the day in German. They often walk home with English speaking friends. And I've noticed that when all the kids are together that they will speak English. But when they are just with a sibling or with me, the conversation is all in German. I've noticed that even when I am not a part of the conversation, that they get off the bus and immediately speak German to each other. I'm anxious to know what language they use to communicate with each other while they are at school. I don't see them at recess and I'm not even sure if they play that much together. I think Simon plays with the boys and Clarissa plays with the girls. But if Simon needs to ask Clarissa a question during class (they are in the same Kindergarten class), what language does he use? I think it's time to do some sleuthing so that I can get some linguistic answers. I'm curious to know if they're using their German at school...