No more ridiculous than blaming the parents, who were at work at the time, for what their kid was doing in the school.
Except that the school *did* tell the parents. (Probably while telling them that their kid is suspended.) And the parents grounded their little bundle-of-joy for a week, so obviously they agree at some level that their kid's a little shit.
Where they dropped the ball is that Little Timmy didn't have to go over to this kid's house and apologize to her face. Not to mention checking to make damned sure that the site was down. If Timmy had sprayed graffiti all over a house, you wouldn't ground him, but figure "nah, he doesn't need to actually clean it up", right?