I see what you're saying, but...
"Superior just means that it is better than some, higher."
I see "superior" as meaning not just higher than some
; I view it is higher than (almost) all. You're right that a "B" grade is significantly higher than a B-, and it's really a pretty good achievement on a B- curve; but still, a B average will put you in probably the top 30-40%, while I think of "superior" as like top 10%.
When I was younger, "B" meant "good" and "A" meant excellent. Maybe my school thought "good" was not indicative of the level of achievement that a "B" grade suggests?
I kind of agree w. your schools grading characterizations. Superior just means that it is better than some, higher. So a C is superior than a D, and an A is superior than a B. So, if you have a B- curve like my school, then B is superior to most people. So it is appropriate.
On the other hand, excellent tends to imply that someone excels. They do above and beyond what was called for, not just above. I think an A would be reflective of this.
I don't know if people agree. I have nothing to back this up, it is just my personal understanding/interpretation of those terms.