repeating courses at Hogwarts?

[Elizabeth Catherine]

No: HPFGUIDX 137821

Hello all,

While reading Chapter 15 with my kids I picked up on something that 
did not catch my eye in my first solo reading and was hoping for 
some insight. I don't recall seeing this discussed to date.
On page 324 of HBP, US edition, Snape says to Draco:
     "If your friends Crabbe and Goyle intend to pass their Defense 
Against the Dark Arts OWL this time around, they will need to work a 
little harder than they are doing at pres-" (Malfoy cuts him off).

My questions:
Are students allowed to retake OWLs for passing grades and if so, do 
they have to repeat the year or just the exam? Would there be any 
incentive to do so? I'm wondering about Harry and History of Magic 
when he had the vision of Sirius...
If this is not standard practice, is Snape just doing this for 
Slytherins as my nine year old suggested? Does Dumbledore know?
On a related note, if the exams are postponed at the end of HBP, 
what are the thoughts on what the OWL and NEWT students will have to 
do to make them up?

Thanks, 
Liz