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.
