Teacher markbook: Letter grade to numerical value average.

Hi everyone,
I am a teacher and I give letter marks including + and - from E- up to A+. What I'd like is for excel to:
Assign a number to each letter grade mark (I have done a table on a separate page in the spreadsheet to reflect this)
Calculate the average numerical grade based on this information.
Convert that number back into an overall letter grade.

For example: Johnny gets: A, B+, A-, C+, B
So he gets numerically: 13, 11, 12, 8, 10.
The average is 10.8 (rounded to 11) So a B+ overall.