You can create a new group under the assignment section and set the weight of the grade to a zero. To add a new group select the + group button then select the gear to set the new weights.
I believe as long as you don't have the pre and post test set to any weight that there will be no affect the student's score. You will see the grades in the grade book but it will not be apart of their score. I do this with my students every year. They take a pre-test at the begining of the course and a post-test at the end. I set the weight to zero. The students see the grades foo the assignments but the assessments don't count in their grade calculation.
In this example the class has 10 assignments each 10 percent of the grade and a pre-post assessment group worth 0 percent. If you wanted to go one step further you could have the pre and post as separate groups.
Honestly this was the first thing I thought of as well, but the teacher doesn't want to weight all of his categories (and he has several since he's a science teacher), he uses total points, so a 0% weighted group was something we were trying to avoid.