There's also the times we see Ranma studying
Manga only again?
or reading in his spare time
That says nothing about his academic abilities. It would only give actual info on it if we either saw him reading lots of stuff that should be above his educational level, or for a negative outlook if he was noticed as... let´s say reading comics by only looking at the pictures.
All in all, I would characterize Ranma as average/above average academically and concerned about having bad grades reflecting badly on him.
He states in grand manner that test scores is completely useless to him... Blatant lack of interest.
It´s repeatedly inferred that he´s (far) behind Akane and some teachers treat him somewhat like a yankee(ie someone who doesn´t care about grades or is only there to sit things out).
Again yes, i rely on anime mostly, but now also the chapter you linked to in the other thread and a little more of the manga.
And the way he instantly assumes the principal is correct about his "horrible" scores shows that he thinks he has reason to believe such to be for real, this rather clearly shows that he is probably used to NOT getting
good scores but apparently doesn´t care much if his usual scores would be shown, because test scores doesn´t
really matter.
If he didn´t think his scores
could be so dreadfully low(if he was in any way used to getting good scores), then he would have called Kuno a liar or some such instead of suddenly becoming scared about it.
If he truly cared about getting high scores, he would never say the very opposite.
Even if relying only on the linked manga chapter, i have a very hard time seeing how you can peg him as an above average student...
Ranms' education I know has been discussed here before in the past, but whatever. In any case he has to be smarter than he seems...
Oh i would say he seems pretty smart, no idiot would be able to fight like he does or copy/improve other peoples moves like he does. However, smart does not automatically have any direct connection with his ability as a student.
And i have a very hard time seeing him as a good student.