Green’s Dictionary of Slang

bang n.5

[initial letter]

(US campus) a grade of B; thus bang and a half, a grade of B+.

[US](con. 1964–8) oral testimony in Lighter HDAS I 368/1: When I was in college an A was an ace, a B was a bang, a C was a cat or a hook, a D was a dog, an F was a frog. B+ was a bang and a half.