Suppose I have letter grade data in a range of cells, B12:D12 as follows
A-, B-, B+/A-

I have another range of cells, named GradeList that gives numerical
equivalents for each of these letter grades. I want a function that will look
up each of these grades in the table, and return their sum, average etc.

I tried the following as an array formula:

{=SUM(VLOOKUP(B12:D12,GradeList,2,FALSE))}

And it didn't work; looks like it just returns the value of the lookup of B12.

Actually, I want this formula to sum the highest N grade values; I know how
to use an array formula to do this with numerical data in cells. But figuring
out how to do the VLookup on each letter grade cell, without having to use
separate cells to contain the numbers, and then sum on numerical equivalents
would be really helpful.