I'm not totally sure whether you're asking for British school info, Lilu, or Hogwarts info. Harry cetainly gets his essays marked with expected OWL grades.
For the Muggle world, the examination system is something like this
GCSE's (equivelent of OWL's) are graded A-G. A-C are the top grades obviously and there the ones that in the old days (HA!) would have been classed as pass rates. JK would write with these in mind which is why she has three pass grades (O, E and A) - the rest are fails. You can fail a GCSE.
A'levels are the equivalent of NEWTS - they are graded A-E. All are pass grades. An F is a fail. Well, that's what it was in my day, I'm not totally sure whether that's still true today.
I think different schools mark things differently. You can have simple A,B, C type grades or A+ A-, B+, B, B-. I used to get essays marked as A= (A double minus) which meant it wasn't quite an A- but was better than a B ++ (double plus) . It used to get very confusing when some teachers marked you as A-(-), (A minus, bracket minus). I think it's simpler now - LOL.
And to confuse things even furthur, my daughter tells me that she gets a number and a letter for her grading. She's year seven and the highest grade you can get is 8A and the lowest 1C.