The class average was .or is ... ? Thanks!

The class average was...or is ...

I have a related question. And the more I think about it, the more I get confused. Yes, the test was done, but if the average is a fact/truth, shouldn't we use "is", instead of "was"?

The test was done last year. The class average is/ was 88. ( I think we can use "is" since fact of the average being 88 remains. However, I sort of see why "was" may be used in this case. The test was one year ago any way)

But, in the following two cases, especially the last one, shouldn't we use "is 88 ?" 

The test was done yesterday. The class average is / was 88. 
The test was done in the morning. The class average is/was 88.

Any help is appreciated!

I have to admit that I find English as a language becomes less and less structured.