It's a tough situation. I think they were shocked that Jeremy had the explosive reaction he had, because in all honesty, Jeremy's not very close to Brad and Jeanine was always the parent he was closest to. (Will went to Brad, Darius went to Robbie, and Jeremy went to Jeanine.) I think they might have figured that Jeremy would have a somewhat blase reaction to it. If I'm playing shrink, though, I think Jeremy's reaction was this bubbling mix about the rejection of his biological father, the truly nasty origins of how he was conceived, and feeling justification of, "Okay, so this is why I've never been Brad's favorite son- I never had a chance to be because I'm not really his son!" Then this got directed towards Will, the Golden Boy who couldn't be anything but Brad's son, and then the nastiness came out of Jeremy's mouth. And damn is Jeremy paying for that.
Also, just general Theater Kid theatrics. Having gone to a performing arts high school, I've known and met a ton of kids like Jeremy. People hit their limit when dealing with kids like this and Jeremy is learning that lesson the hard way.