Simple would be to use =RandBetween(desired average + 10, desired average -10) in each of the 24
assignement grades.

For example,

=RANDBETWEEN(73,93)

The numbers could be in cells that are referenced:

=RANDBETWEEN($A2,$B2)

HTH,
Bernie
MS Excel MVP

Sub FindNumbersThatAverage()
' Provides random numbers that average a predetermined amount.
' Jim Cone - San Francisco, USA - May 29, 2005
Dim lngN As Long
Dim lngLow As Long
Dim lngTemp As Long
Dim lngHigh As Long
Dim lngTarget As Long
Dim lngQuantity As Long
Dim lngArray() As Long

'Establish parameters...       '<<<<<
lngLow = 70
lngHigh = 100
lngTarget = 83
lngQuantity = 24

'Sanity check
If lngLow > lngTarget Or lngHigh < lngTarget Then
Exit Sub
End If
'The number of numbers must be an even number <g>
If Not lngQuantity Mod 2 = 0 Then
lngQuantity = lngQuantity + 1
End If

ReDim lngArray(1 To lngQuantity)

For lngN = 1 To lngQuantity Step 2
'Get random values between the high and low parameters.
Randomize lngTemp
lngTemp = Int(Rnd * (lngHigh - lngLow + 1)) + lngLow

'Assign random values
lngArray(lngN) = lngTemp
lngArray(lngN + 1) = 2 * lngTarget - lngTemp

'If the high/low range is not centered on the target average
'then the random results may need adjusting.
If lngArray(lngN + 1) > lngHigh Then
lngArray(lngN) = 2 * lngTarget - lngHigh + lngN
lngArray(lngN + 1) = lngHigh - lngN
End If
If lngArray(lngN + 1) < lngLow Then
lngArray(lngN) = 2 * lngTarget - lngLow - lngN
lngArray(lngN + 1) = lngLow + lngN
End If
Next 'lngN

'Stick it on the worksheet.
Cells(Rows.Count, 1).End(xlUp).Offset(1, 0).Resize(1, lngQuantity).Value = lngArray()
End Sub

One of a few ways...
Under Data Analysis, one can select "Random Number Generation."
Select a "Normal" distribution, and set a mean (83%), and a Standard
deviation of your choice.
Takes about 2 seconds in Excel 2003, and over 1 hour in Excel 2007.  That's
how long it took me to find it.  Excel 2007 is simply the worst program I
have ever seen.