I think both are correct

The test was done yesterday. The class average is 88.  This emphasizes the fact: the average is a fact.

The test was done yesterday. The class average was 88.  This emphasizes what happened yesterday.
It also depends on the context, if you emphasize a fact or an event.

You could use the same principle to explain the is/was pair in the following sentences.

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

I might be wrong.