The Weasleys (update on JKR's page)

[potioncat]

No: HPFGUIDX 115098


 
> Finwitch:
> 
> Fred could well be keeping George company. It's not like he has 
much 
> *else* to do, nor would he wish to spend time without his twin. 
And 
> Fred might be getting ingredients for their Products while George 
> attends the class... and I'm not certain if Sprout required an 
> accepted OWL for that class. (and George didn't pass his 
theoretical 
> Herbology either).
> 
> It's even possible that the two were having *private* lessons from 
> Sprout! (Remember that ALL of the staff was anti-umbridge, so 
> Fred&George may well have persuaded the teachers to give them 
lessons 
> for anti-umbridge pranks...)
> 
Potioncat:
You are probably right. It is very likely that a student wouldn't 
have to pass OWLs to take Herbology in 6th and 7th years.  After 
all, McGonagall makes a point that "she" requires and E and 
that "Snape" requires an O, not that the class requirement is for 
those marks.

It fits with the Hufflepuff philosophy of teaching them all.  And we 
were told in SS/PS that the twins got good marks.  So class grades 
might play a part in this too.