Turns out that an A is the most common grade given at Harvard College. Clearly, then, only an A+ would signal somewhat exceptional achievement. The elite schools are all "A" schools now except in math and the hard sciences. After all, you pay lots of money for these distinctions.
Here's a good idea: Indicate grade and average grade for the class.
However, I do understand the challenge of grading six or seven very smart, avid kids in a Chaucer seminar, each one of whom contributes interestingly and each one of whom memorizes verses in Old English and writes a fine, inventive, well-structured, and perfectly-grammatical essay on some aspect of the Canterbury Tales or The House of Fame.
Of course, any demanding prep school would expect the same.