The study used a "diffs-in-diffs" technique which would control for that. It takes the difference between two groups, say English and Dutch, prior to banning ma

The study used a "diffs-in-diffs" technique which would control for that. It takes the difference between two groups, say English and Dutch, prior to banning marijuana for English students, and then looks at the difference between the two groups afterwards. So it's a way of looking at how the performance of the English group changed with banning the drug, while trying to control for the things that affected _all_ students.