I don't think anyone could give you an opinion that would be accurate, BFA. Just like with most cases, it'll depend entirely on that court/judge and on how good your boyfriend's attorney is, assuming he has one.

I tend to agree with your mom about staying home. Even if you were to go, he's going to be called to appear before the court with his mom and his attorney and no one else, so you wouldn't be standing up with him anyway or be nearby during the proceedings. You'd just be sitting in the background someplace.

If you are fine with that and want to go simply for moral support and to see him in case they cart him off in handcuffs after the proceedings, then dress conservatively. I suspect you're too young to own a blue suit, but I hope he has one. Seriously, just be conservative and not flashy, spiky, boheme, over-jeweled/pierced or short-skirted. Go easy on the eyeliner. It's about respect for the court, really, and not about appearing a certain way (other than as an upstanding, clean-living local youth, of course).