Suspension in boarding schools

[Brian]

Replying to
 > If the kid doesn't do the work, he fails the class. (It doesn't seem 
to work that way at Hogwarts, though.)

Keeping someone back a year is almost unheard of in British state (i.e. 
public) schools, that's why.  I've never heard of it happening.  I can't 
speak for public (i.e. private) schools.  However, apparently it 
happened to Marcos Flint, although JKR only said that he'd been kept 
back a year when it was pointed out that she'd had him there for eight 
years, so I suspect it was an excuse for an error rather than her intention.

Replying to
 > Carol, who still wonders what suspension amounts to at Hogwarts since 
we never see it (even the expelled Hagrid remains on the school grounds)

I think Hagrid remaining on the grounds was very much the exception 
because the great Dumbledore wanted him to stay, even though, as he 
wasn't Headmaster, he couldn't stop his expulsion.

Brian