Home friends, school enemies

I have a very social phobic 8th grader and a far too social 6th grader. They can't really be in the same room together without lobbing comments at each other because they make each other so anxious. I always try to avoid having them in the same place at the same time but today there was some overlap. So one started to say something about the other being crazy and the other says something about hurting the first one, etc, etc. I got the 6th grader calm enough to go to his next class which quieted the 8th grader down. However, he was still too anxious to do any work so we just talked for 20 minutes.

When I asked him about why he doesn't like the 6th grader he told me that they get along fine when P. is there. I asked him to explain and he told me that the 6th grader and their mutual friend P. often stop by his house on the weekends and they all play or hang out or whatever. It turns out that they all live quite near each other and spend a lot of time together away from school. I found this all so strange- they're buddies at home but foes at school. What could cause this?

My hypothesis is that they do things at home which would get them into trouble at school and that makes them nervous. Also, the 6th grader is relatively popular while the 8th grader isn't. It wouldn't be "cool" to be seen hanging out in school together. Or something like that. Though it's my job to seek to understand the minds of these students, in social situations like this, they remain obscure.