The version I gave you is 2003 compatible. Is there a specific reason it has to be in a single cell? The only way to get all the results in a single cell would be either a VBA solution which means they would have to enable macros each time they opened the file, or using an addin like MoreFunc and they would have to have the addin installed also, or using the provided results as helper columns (details below).
The only text joining function, Concatenate, does not work with arrays, so there is no built in solution to get all of the results into a single cell.
You could, however, use the formula I provided to get the proper results off to the side as helpers, and then use a formula similar to this to bring in the results to a single cell, as long as you don't mind having a lot of helpers (whose columns can be hidden):
So assuming the unique list of grade combos is in column D, and the helpers are in column N as shown in the file I posted, this would be the formula in cell E2 and copied down:
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