Does Smoking Weed Really Lead To Lower Grades?

A new, recently released study has shown that college students who use marijuana get 5% lower grades than those that don’t use the drug.
A segment on The Young Turks pointed out a couple of issues with the study, including the fact that it compared a college where students had legal access to marijuana, to a college where students are banned from buying legal marijuana.
Just because those students were banned doesn’t mean they weren’t buying it elsewhere and smoking up.
Does this study prove that marijuana should be illegal?