Historical Reporting On Changing Data

I've got a customer who wants reproducible/historical reporting. The problem is that the underlying data changes.

I tried to explain that this can't be done (can it?), but he doesn't
understand.

To illustrate the situation - Let's say a teacher wants to track
spelling test scores for her students.
The below are scores for students A, B, and C (for January, February, March)

A: {70,80,85}
B: {70,65, 80}
C: {100,90,100}

So, I can generate a historical report that charts the class average
and student trend - that's pretty easy.

Now, in April, we find that the school board has mandated that the
British spelling of words is ok, so now the cumulative scores (for
January, February, March, April)

A: {90,80,85,100}
B: {80,65, 80,80}
C: {100,90,100,75}

He wants a report showing the January average as (70+70+100)/3 = 80,
when really it is (90+80+100)/3 = 90.

Now imagine that there are actually thousands of data points changing like this...
Now also imagine that we add and remove students on a regular basis...

He and his office manager get frustrated when I explain that the
reports are not simple - in their mind it is. They have determined
the solution is to get a report writer and buy Crystal Reports...
I've tried to explain that the problem is that the report
specification is unclear (basically - they don't understand what they want). The situation is ok for now, I'm just trying to plan for when they figure out that buying Crystal Reports won't change their situation (except they are done several thousand dollars)...

Any tips?