Do not hyphenate grade point average or put periods in its abbreviation, GPA. GPAs refer to numbers, not grades.
a GPA of 3.0, not a GPA of B
When referring to a grade, use a capital letter; quotation marks should not be used around letter grades. Use an en dash to indicate a minus sign.
a B– average for the course, a P/F course, a grade of I (Incomplete)
Pluralize single letter grades with apostrophes.
She got mostly B’s and C’s all year.
See also capitalization.