Calculating the weighted average of letter grades

Hi all,

I'm something of an Excel neophyte, and while I've mostly been able to cobble together some clunky algorithms to do what I need, I'm stuck on one point which has a few more variables than I can sort through, and have come up dry after scouring the internet.

Basically, I'm preparing a grading spreadsheet into which my TAs can enter student grades for each individual student and each assignment.
That looks a bit like this:

Midterm Paper Final Exam Course Average
Student 1 A B+ A-
Student 2 A- B B+

<tbody>
</tbody>

I want to find out how to get them a weighted average letter grade for the final column.

For each of 7 assignments, students receive letter grades with +s or -s.

<style type="text/css"><!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--></style>
A 1
A- 2
B+ 3
B+ 4
B- 5
C+ 6
C 7
C- 8
D+ 9
D 10
F 11

<tbody>
</tbody>

Each assignment is weighted differently:

<style type="text/css"><!--td {border: 1px solid #ccc;}br {mso-data-placement:same-cell;}--></style>
Assignment Weight factor
Midterm 1 10%
Midterm 2 10%
Paper 25%
Participation 20%
Final 1 11.67%
Final 2 11.67%
Final 3 11.67%

<tbody>
</tbody>


Because the weight factors and number of grade values are a bit complicated, I'd love it if Excel could average this for them.
Any insight into how I can make this happen would be hugely appreciated.

Please attach a sample workbook.