The great brooder war

So I have a large greco travel playpen set up as a brooder right now with a piece of screen that seperates to seperate parts... on one side are my 6 3 week old pullets (2 BR, 2 buff chants 1 red leghorn and 1 partridge chant) on the other side are 6 5 day old bantam chicks. There is a screen inbetween. They are together 24/7 and yet if I try to remove the screen that seperates them, the older pullets literally beat on the bantams. Anyone have any ideas how I can get these guys to get along? They are perfectly calm when I have the screen in place, this is what makes no sense...