The grade itself has a + or -, so it is clear to everyone that it's about 'fine-tuning'. If you translate it, it will sound funny. I'd just leave it out.

Alternatively you could say 'grade, fine-tuned', but only if you REALLY insist.

If you look at the table provided by Wikepedia, you will see that 3P in the Oberstufe is equivalent to the customary school mark 5 with positive leaning (i.e. 5+), 7P to a 3 with a negative leaning (3-)and 8P is a 3 with neither positive nor negative leaning (we call this "eine glatte Drei").
In fact the official marks in the Oberstufe are the points which are all added up to a grand total of so many out of 900 (well, that was our system in Bavaria until recently), so I would suggest only referring to the points.
It is certainly easier to understand that 8 out of max 15 is better than 6 than to explain exactly how much better 3+ is than 3 or 3- (or the fact that "3 leaning towards 4" is better than "upright 3" which is itself better than "3 leaning backward towards 5".
Sorry if I'm getting facetious, but I really think it is easier not to try any translation of "Tendenz".


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Note added at 9 hrs (2010-02-08 07:25:44 GMT)
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Oh dear, on re-reading my entry I discover that I am only adding to the confusion. My maths is all wrong. My last-but-one sentence should read: "It is certainly easier to understand that 9 out of max 15 is better than 7 than to explain exactly how much better 3+ is than 3 or 3- (or the fact that "3 leaning towards 2 "(=9P) is better than "upright 3" (=8P) which is itself better than "3 leaning backwards towards 4"(7P) Sorry again !