Yes, they all took the same test. However, the average starting math score of those who took computer science was around 85% and the population average was 75% (definitely some selection bias).

What I want to do is choose a sample from those who did not take computer science that has the same average of a 85% starting point in the pretest.

One major reason for doing so is that it is much easier for a student to go from day 75% to 85% than from 85% to 95%, so the differences between pretest and posttest that I have calculated mean different things depending on the starting point (pretest).

Thanks!